Method for purifying (meth)acrylic acid

ABSTRACT

A method for purifying a crude (meth)acrylic acid obtained by a vapor phase catalytic oxidation method, characterized in that the crude (meth)acrylic acid having most parts of water and acetic acid removed therefrom, is fed to and distilled in a first distillation column of a purification system comprising first to third three distillation columns, the top fraction from the first distillation column is fed to and distilled in the second distillation column, the resulting top fraction is recovered as a high purity (meth)acrylic acid product, the bottoms from the first and second distillation columns are fed to and distilled in the third distillation column, and the resulting top fraction is fed to the first distillation column.

This is a continuation application of U.S. application Ser. No.10/834,075, filed Apr. 29, 2004, which is a continuation ofPCT/JP02/11308 filed on Oct. 30, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for purifying (meth)acrylicacid, particularly to a method for purifying a crude (meth)acrylic acidobtained by vapor phase catalytic oxidation, by distillation to obtainhighly pure (meth)acrylic acid which is useful for the production of ahighly water absorptive resin and for the production of a (meth)acrylicester. In this specification, (meth)acrylic acid means acrylic acid ormethacrylic acid.

2. Discussion of Background

a. As a method for producing (meth)acrylic acid, a method of hydrolyzingthe corresponding nitrile compound may, for example, be mentioned.However, at present, a vapor phase catalytic oxidation method of thecorresponding hydrocarbon such as propylene or isobutylene, is mainlyemployed. Recently, a study has been made also on a vapor phasecatalytic oxidation method using an inexpensive corresponding alkane asthe starting material instead of an olefin.

In the production of (meth)acrylic acid by a vapor phase catalyticoxidation method, firstly the reaction product gas containing(meth)acrylic acid is contacted with an absorbing solvent such as waterto recover (meth)acrylic acid in the gas in the form of a (meth)acrylicacid solution. This solution contains, in addition to (meth)acrylicacid, various impurities formed as by-products during the vapor phasecatalytic oxidation, such as acetic acid, maleic acid, acrolein,furfural, benzaldehyde, acetone, etc. in the case of acrylic acid. Manymethods have been proposed for recovering purified (meth)acrylic acidfrom such a (meth)acrylic acid solution. However, the principal ones aresuch that the absorbing solvent and a part of impurities are removedfrom the (meth)acrylic acid solution in a preliminary purification stepto obtain a crude (meth)acrylic acid substantially comprising(meth)acrylic acid, dimers thereof and other heavy components, and thensuch a crude (meth)acrylic acid is purified in a purification step toobtain a product having a desired quality.

b. For example, in recent years, acrylic acid has found an increase inits demand as a starting material for e.g. a food additive or a highlywater absorptive resin for e.g. paper diapers. In such applications,highly pure acrylic acid is required. Namely, if crude acrylic acid isused as a starting material for a polymer of acrylic acid withoutremoving impurities, there will be a problem such as a delay in thereaction during the polymerization reaction, a decrease in thepolymerization degree or coloring of the polymerized product.

Accordingly, industrially, purification of acrylic acid is carried outby distillation. However, it is not easy to remove by distillationimpurities in crude acrylic acid obtained by vapor phase catalyticoxidation.

Heretofore, as a method for producing highly pure acrylic acid byseparating and removing impurities from crude acrylic acid obtained byvapor phase catalytic oxidation, a method of carrying out distillationin the presence of a hydrazine, has, for example, been known(JP-A-49-30312, JP-B-58-37290, etc.). However, such a method isprimarily intended to remove an aldehyde in the crude acrylic acid,whereby removal of maleic acid and/or maleic anhydride (these will betogether hereinafter referred to as “maleic acids”) tends to beinadequate.

Further, JP-A-7-330659 discloses a method of carrying out distillationin the co-presence of hydrazine and ammonia. This method is effectivefor removal of maleic acids, but has a problem such that the addedammonia will be distilled from the top, such being not suitable for theproduction of highly pure acrylic acid. Further, this applicationdiscloses batch treatment only, and discloses nothing about a method forcontinuously obtaining highly pure acrylic acid on a commercial scale.

Accordingly, it has been considered that with these techniques, it isnot easy to continuously produce high purity acrylic acid bysufficiently removing impurities containing maleic acids from the crudeacrylic acid.

On the other hand, JP-A-2001-316326 discloses a method for continuouslyproducing high purity acrylic acid by preventing sludge formation in thedistillation column, wherein crude acrylic acid having a concentrationof maleic acids of at most 2000 ppm, is used as a starting material forhigh purity acrylic acid. However, in order to reduce the concentrationof maleic acids in the starting material crude acrylic acid, it isnecessary to remove maleic acids in the step of obtaining the crudeacrylic acid, and such does not provide a substantial solution to theproblem. Further, in the process for producing acrylic acid, forexample, in a step of recovering acrylic acid from the bottom residue ofthe acrylic acid distillation column, maleic acids, will be distilled inacrylic acid, and consequently, accumulation of maleic acids will takeplace within the acrylic acid production process, and accordingly, fromthe industrial view point, it is desired to develop an economicallyexcellent method whereby crude acrylic acid containing at least 2000 ppmof maleic acids can be used as the starting material, and yet, highpurity acrylic acid can be continuously produced constantly.

c. On the other hand, as a purification method for (meth)acrylic acidobtained by vapor phase catalytic oxidation of propylene or isobutylene,a distillation method is common, but (meth)acrylic acid is extremelysusceptible to polymerization, and its handling was problematic.

d. As one of distillation apparatus, a vertical thin film evaporator isknown which comprises an evaporator main body with its principal portionbeing cylindrical, which has a heating means on its exterior surface, aliquid inlet and a vapor outlet at its upper portion and a residuedischarge port at its lower portion, a rotary shaft set in the mainbody, and stirring vanes attached to the shaft and being movable in aperipheral direction along the inner wall surface of the evaporator mainbody. With this thin film evaporator, the interior surface correspondingto the exterior surface on which the heating means is provided, is aheat transfer surface, and a liquid to be treated, which is suppliedfrom the upper liquid inlet will be pressed and spread in a film form onthe cylindrical inner wall surface by the rotating stirring vanes, andin the process where this liquid film falls by gravity, low boilingpoint components in the liquid to be treated are permitted to evaporateby the heat supplied from the heating means. This apparatus is capableof evaporating low boiling point components in the liquid to be treated,in a short time, and thus, it is suitable for treating a liquidcontaining a substance sensitive to heat, such as areadily-polymerizable compound. Further, the treated liquid is forciblystirred by the rotating stirring vanes, whereby the liquid in contactwith the heat transfer surface is always renewed by a fresh liquid,whereby there is a merit such that local overheating of the liquid to betreated can be prevented, and baking or scaling of the liquid tends toscarcely occur.

Many methods are available for attaching stirring vanes to the rotaryshaft. For example, in a movable vane system, the stirring vanes areattached to the rotary shaft via fulcrums or springs, so that they canbe moved in a circumferential direction about the rotary shaft, wherebyby rotation of the rotary shaft, they rotate while contacting with thecylindrical inner wall surface or while maintaining a slight distancetherefrom, by a centrifugal force.

e. On the other hand, heretofore, it has been common to employ a methodfor producing an acrylic ester by an esterification reaction of acrylicacid with an alcohol. As the acrylic acid to be used, one obtained by avapor phase oxidation reaction of propylene, followed by dehydration,removal of low boiling point impurities and further purificationtreatment for removal of e.g. high boiling point impurities, may beused. However, it has been regarded advantageous to employ one notsubjected to treatment for removal of high boiling point impurities,since purification costs of acrylic acid can thereby be made low(JP-A-9-157213, JP-A-10-237012, JP-A-10-306052, JP-A-2001-213839).

However, if acrylic acid containing high boiling point impurities, isused as the starting material, there have been problems such thatundesirable polymerization reactions or side reactions are likely totake place, thus leading to clogging of apparatus such as pipes bypolymerized products, deterioration of unit consumption of mainmaterials such as acrylic acid and an alcohol, and a decrease in thequality of the product.

f. The acrylic acid-containing gas obtained by vapor phase oxidationwill then be contacted with water in a collection column to obtain anaqueous acrylic acid solution, and an azeotropic agent is added to thisaqueous acrylic acid solution, whereupon in an azeotropic agentdehydration distillation column, an azeotropic mixture comprising waterand the azeotropic agent, is distilled, while crude acrylic acidcontaining acetic acid is recovered from the bottom of the column. Then,this crude acrylic acid is subjected to a distillation column forseparating low boiling point components thereby to separate low boilingpoint impurities such as acetic acid, and further, high boiling pointimpurities are removed in a distillation column for separating highboiling components, to obtain purified acrylic acid. Further, there maybe a case where acrylic acid is collected by contacting it with a highboiling point solvent in a collection column.

Acrylic acid thus produced, may be used as a starting material forvarious acrylic esters. In recent years, its demand as a startingmaterial for a highly water absorptive resin has increased. Such acrylicacid as a starting material for a highly water absorptive resin isrequired to be acrylic acid purified to a high purity, and especially,aldehydes are required to be highly removed, since they tend to hinder apolymerization reaction or they tend to color the product polymer.

Heretofore, as a method for removing aldehydes simply and efficientlyfrom purified acrylic acid, a method is known wherein aldehydes areconverted to heavy substances by means of an aldehyde-removing agent ofe.g. an amine system including a hydrazine system or an amino acidsystem (JP-A-49-30312, JP-A-49-95920, JP-B-50-14, JP-A-10-204024), ahydrogen sulfite system (JP-A-7-330672), a mercaptan system(JP-A-60-6635) or a combined system of a hydrazine system and adithiocarbamate system (JP-A-7-228548), followed by distillation in adistillation column for purification, to obtain high purity acrylic acidfrom the top of the distillation column for purification.

The bottom fraction containing high boiling point compounds formed atthe time of this distillation, contains, together with reaction productsof aldehydes with an aldehyde-removing agent, a polymerization inhibitorsuch as hydroquinone added at the time of the distillation, and furtherhigh boiling point substances formed during the distillation, such asmany heavy substances, such as an acrylic acid dimer(β-acryloxypropionic acid) or oligomers being Michael adducts of acrylicacid, polymers, etc.

Heretofore, such bottom fraction was disposed, or recovered for theproduction process for acrylic acid. This bottom fraction containsacrylic acid dimer, etc. being Michael adducts, and if it is recoveredfor the production process for acrylic acid, it is considered preferredto treat it in a thermal decomposition treatment step before recovery inthe purification step for acrylic acid (JP-A-2001-213839).

g. It is an object of the present invention to solve the above-mentionedconventional problems and to provide a method for producing high purity(meth)acrylic acid by sufficiently removing impurities such asaldehydes, ketones, dicarboxylic acids such as maleic acids from crude(meth)acrylic acid obtained by a vapor phase catalytic oxidation method,which is an economically excellent method for producing (meth)acrylicacid, whereby continuous operation for a long period of time is possiblewhile suppressing formation of sludge in the distillation column.

h. Further, as mentioned above, a high purity (meth)acrylic acid productis required as the starting material for a water absorptive resin suchas paper diapers. The reason is that if an impurity, particularlyfurfural, is contained in the above-mentioned (meth)acrylic acidobtained by vapor phase catalytic oxidation, there will be a problemsuch as delay in the reaction, deterioration of the polymerizationdegree, coloration of the polymerized product, etc. at the time of thepolymerization reaction for a water absorptive resin. Therefore,industrially, purification of (meth)acrylic acid is carried out bydistillation or crystallization. Crystallization usually requires alarge initial investment, and from the economical viewpoint, a method bydistillation is employed in many cases, but it is difficult to removethe above impurity, particularly furfural, by usual distillation.

In order to solve this problem, a method has been proposed wherein ahydrazine compound is added at the time of purification of (meth)acrylicacid. This method is effective from the viewpoint of removal of theabove-mentioned impurity, but has had a problem that it causespolymerization of (meth)acrylic acid during the rectification.

Formation of a polymer causes clogging in the distillation column,whereby the performance of the distillation column decreases, or it willbe required to stop the operation. Accordingly, a method for suppressingformation of such a polymer, is desired.

JP-A-7-228548 proposes to suppress the formation by adding copperdithiocarbamate. In an operation for a short time, the effect of thismethod is confirmed, but in a continuous operation for a long time for ausual industrial operation, the effect has been still inadequate.

i. Further, in the purification of (meth)acrylic acid or its ester bydistillation, if it is attempted to recover (meth)acrylic acid or itsester by means of a thin film evaporator from a heavy componentcontaining (meth)acrylic acid or its ester discharged from the bottom ofthe distillation column, there is a problem such that cloggingfrequently occurs at a liquid-withdrawal tube or at an outlet portion ofa liquid collection part of the thin film evaporator.

The present inventors have sought to find out the causes and as aresult, have found them to be such that the liquid flowing down on theinner wall surface of the thin film evaporator will polymerize on alower inner wall surface rather than at the lower end of the stirringvanes, and as the liquid introduced into the same film evaporator isconcentrated, a polymerization inhibitor preliminarily added for(meth)acrylic acid or its ester, will precipitate.

Namely, in the thin film evaporator, stirring vanes are disposed to stirthe liquid film on the heat transfer surface where evaporation takesplace, and no stirring vanes are disposed at the inner wall surfaceportion below the heat transfer surface, particularly at the invertedcorn-shaped liquid collection portion following the cylindrical portion,or at the funnel-shaped liquid collection portion being a combination ofthe inverted corn-shape and a cylindrical shape. Accordingly, at such aportion, the liquid flowing down, has the majority of low boiling pointcomponents removed and thus essentially has bad fluidity, and besides,no stirring by stirring vanes takes place, whereby the liquid in contactwith the inner wall surface tends to be hardly renewed by a freshliquid. Consequently, the retention time of the liquid in contact withthe inner wall surface tends to be abnormally long, and (meth)acrylicacid or its ester remaining in the liquid, tends to gradually polymerizeto change the liquid to be heavy, whereby the fluidity of this liquidfurther decreases, and a polymer tends to accumulate on the inner wallsurface. Further, as the liquid is concentrated, the polymerizationinhibitor preliminarily added for (meth)acrylic acid or its ester tendsto be precipitated. The accumulated polymer and precipitates not onlyhinder the flow of the liquid, but also clog the outlet portion of theliquid collection portion or the following liquid withdrawal tube, ifthey are peeled off from the inner wall surface. Accordingly, it is anobject of the present invention to provide a thin film evaporator freefrom such clogging.

j. It is an object of the present invention to avoid conventionalproblems such as clogging of apparatus such as pipes by a polymer,deterioration of the unit consumption of the starting materials,deterioration of the quality of the product, etc. and to provide amethod for producing an acrylic ester which is economically excellentand industrially advantageous.

k. Further, it is not desirable to obtain high purity acrylic acid fromthe top of the above-mentioned distillation column for purificationwhile subjecting the bottom fraction of the distillation column forpurification to thermal decomposition treatment to recover it for anacrylic acid purification step, because as the thermal decompositiontreatment is carried out at a high temperature, many side-reactions ordecomposition reactions will take place, whereby formation ofundesirable by-products which cause to accelerate polymerization ofacrylic acid, to contaminate an acrylic acid product or to presentcoloration to the product, or regeneration of aldehydes, takes place,and such compounds are likely to be recycled to the purification step ofacrylic acid.

To avoid such problems, a method has also been proposed whereindistillation is carried out under such a distillation condition that theacrylic acid concentration in this bottom fraction will be sufficientlylow, and the bottom fraction is subjected to disposal treatment.However, if the concentration of acrylic acid in the bottom fraction islowered, the viscosity of the bottom fraction will increase, wherebyprecipitation of a polymer, etc. tends to readily take place, thusleading to a trouble of clogging at the withdrawal pipe. Accordingly,the lowering of the acrylic acid concentration is limited, whereby ithas been impossible to avoid a loss of acrylic acid in an amountcorresponding to the one disposed as contained in the bottom fraction.

It is an object of the present invention to solve the above describedconventional problems and to provide a method for producing high purity(meth)acrylic acid, whereby a highly purified high purity (meth)acrylicacid is produced by simply and efficiently removing aldehydes containedin (meth)acrylic acid, and at the same time, a waste liquid other thanhigh purity (meth)acrylic acid fraction formed by this treatment ofaldehydes, is recovered industrially advantageously.

SUMMARY OF THE INVENTION

The present inventors have conducted various studies to solve theabove-mentioned problems and as a result, they have found it possible toaccomplish the above-mentioned objects and have arrived at the inventionhaving the following characteristics.

(1) A method for purifying a crude (meth)acrylic acid obtained by avapor phase catalytic oxidation method, characterized in that the crude(meth)acrylic acid having most parts of water and acetic acid removedtherefrom, is fed to and distilled in a first distillation column of apurification system comprising first to third three distillationcolumns, the top fraction from the first distillation column is fed toand distilled in the second distillation column, the resulting topfraction is recovered as a high purity (meth)acrylic acid product, thebottoms from the first and second distillation columns are fed to anddistilled in the third distillation column, and the resulting topfraction is fed to the first distillation column.

(2) The method according to the above (1), wherein the top fraction fromthe first distillation column is, after applying an aldehyde removaltreatment thereto or after adding an aldehyde removing agent thereto,fed to the second distillation column.

(3) The method according to the above (1) or (2), wherein the aldehyderemoval treatment comprises adding a hydrazine as an aldehyde removingagent and heating to a temperature of lower than 80° C.

(4) The method according to any one of the above (1) to (3), wherein inthe second distillation column, distillation is carried out at a columnbottom temperature of at most 110° C. in the presence of a hydrazinecompound and a polymerization inhibitor comprising copper (meth)acrylateand copper dithiocarbamate.

(5) The method according to any one of the above (1) to (4), wherein asthe third distillation column, a vertical thin film evaporator isemployed which comprises an evaporator main body with its principalportion being cylindrical, which has a heating means on its exteriorsurface, a liquid inlet and a vapor outlet at its upper portion and aresidue discharge port at its lower portion, a rotary shaft set in themain body, and stirring vanes attached to the shaft and being movable ina peripheral direction along the inner wall surface of the evaporatormain body, and which has wipers movable in a peripheral direction incontact with the inner wall surface between the lower end of thestirring vanes and the residue discharge port; and the bottoms are fedto the vertical thin film evaporator and permitted to flow down on theinner wall surface, and the resulting vapor of (meth)acrylic acid isrecovered from the vapor outlet at the upper portion.

(6) The method according to any one of the above (1) to (5), wherein atleast a part of the top fraction from the first distillation columnand/or at least a part of the bottoms from the second distillationcolumn, is used as a material for a (meth)acrylic ester.

(7) The method according to the above (6), wherein the (meth)acrylicacid to be used as the material for a (meth)acrylic ester, contains atmost 1,000 weight ppm of β-acryloxypropionic acid, at most 500 weightppm in a total amount of furfural and benzaldehyde, and at most 2,000weight ppm of maleic anhydride.

(8) A method for purifying a (meth)acrylic acid obtained by a vaporphase catalytic oxidation method, characterized in that a crude(meth)acrylic acid having impurities tentatively removed via apreliminary purification step, and a top fraction from a thirddistillation column, are fed to and distilled in a first distillationcolumn of a purification system comprising first to third threedistillation columns, the top fraction from the first distillationcolumn is, after applying an aldehyde removal treatment thereto or afteradding an aldehyde removing agent, fed to and distilled in the seconddistillation column, the resulting top fraction is recovered as aproduct, the bottoms from the first and second distillation columns arefed to and distilled in the third distillation column, the resulting topfraction is fed to the first distillation column, and the bottomfraction is discharged out of the purification system.

(9) The method according to the above (8), wherein as the thirddistillation column, a thin film evaporator is used.

(10) A method for producing an acrylic ester, which comprises reactingan acrylic acid with an alcohol, wherein as the acrylic acid, an acrylicacid is used which contains at most 1,000 weight ppm ofβ-acryloxypropionic acid, at most 500 weight ppm in a total amount offurfural and benzaldehyde, and at most 2,000 weight ppm of maleicanhydride.

(11) The method according to the above (10), wherein the acrylic acid isan acrylic acid obtained by a vapor phase catalytic oxidation reactionof propylene.

(12) A method for producing a high purity (meth)acrylic acid, whichcomprises extracting and/or distilling a reaction product containing a(meth)acrylic acid obtained by vapor phase catalytic oxidation to removelow boiling point impurities and high boiling point impurities from thereaction product thereby to obtain a purified (meth)acrylic acid,treating the purified (meth)acrylic acid with an aldehyde removingagent, and then distilling it in a distillation column to obtain a highpurity (meth)acrylic acid from the top of the distillation column,characterized in that the bottom fraction from the distillation columnis used as a material for producing a (meth)acrylic ester.

(13) The method according to the above (12), wherein the (meth)acrylicester is methyl (meth)acrylate and/or ethyl (meth)acrylate.

(14) A thin film evaporator being a vertical thin film evaporator whichcomprises an evaporator main body with its principal portion beingcylindrical, which has a heating means on its exterior surface, a liquidinlet and a vapor outlet at its upper portion and a residue dischargeport at its lower portion, a rotary shaft set in the main body, andstirring vanes attached to the shaft and being movable in a peripheraldirection along the inner wall surface of the evaporator main body,characterized in that it has wipers movable in a peripheral direction incontact with the inner wall surface between the lower end of thestirring vanes and the residue discharge port.

(15) A thin film evaporator being a vertical thin film evaporator whichcomprises an evaporator main body with its principal portion beingcylindrical and its lower portion constituting an inverted cone-shapedliquid collection portion, which has a heating means on its exteriorsurface, a liquid inlet and a vapor outlet at its upper portion and aresidue discharge port at its lower portion, a rotary shaft set in themain body, and stirring vanes attached to the shaft and being movable ina peripheral direction along the inner wall surface of the evaporatormain body, characterized in that it has wipers movable in a peripheraldirection in contact with the inner wall surface of the invertedcone-shaped liquid collection portion.

(16) A thin film evaporator being a vertical thin film evaporator whichcomprises an evaporator main body with its principal portion beingcylindrical and its lower portion constituting a funnel-shaped liquidcollection portion with a combination of an inverted cone-shape and acylindrical shape, which has a heating means on its exterior surface, aliquid inlet and a vapor outlet at its upper portion and a residuedischarge port at its lower portion, a rotary shaft set in the mainbody, and stirring vanes attached to the shaft and being movable in aperipheral direction along the inner wall surface of the evaporator mainbody, characterized in that it has wipers movable in a peripheraldirection in contact with the inner wall surface of the funnel-shapedliquid collection portion.

(17) A thin film evaporator being a vertical thin film evaporator whichcomprises an evaporator main body with its principal portion beingcylindrical, which has a heating means on its exterior surface while thecorresponding interior surface constitutes a heat transfer surface, andhas a liquid inlet and a vapor outlet at its upper portion and a residuedischarge port at its lower portion, a rotary shaft set in the mainbody, and stirring vanes attached to the shaft and being movable in aperipheral direction along the inner wall surface of the evaporator mainbody, characterized in that it has wipers movable in a peripheraldirection in contact with the inner wall surface being a non-heattransfer surface at the lower portion of the evaporator main body.

(18) The thin film evaporator according to any one of the above (14) to(17), wherein the wipers are attached to the same rotary shaft as therotary shaft to which the stirring vanes are attached.

(19) The thin film evaporator according to any one of the above (14) to(17), wherein the wipers are attached to the same rotary shaft as therotary shaft to which the stirring vanes are attached, and they aremovable vanes.

(20) A method for recovering (meth)acrylic acid or its ester from adistillation residue of (meth)acrylic acid or its ester, characterizedby feeding a liquid containing (meth)acrylic acid or its ester to thethin film evaporator as defined in any one of the above (14) to (19)from the liquid inlet at its upper portion to let it flow down on theinner wall surface, wherein a vapor of the (meth)acrylic acid or itsester formed, is withdrawn from the vapor outlet at its upper portion tooutside, and the distillation residue is withdrawn from the residuedischarge port to outside.

(21) A method for producing (meth)acrylic acid, which comprises feedinga crude (meth)acrylic acid obtained by vapor phase catalytic oxidationto a distillation column to continuously distil and purify it in thepresence of a hydrazine, characterized in that the hydrazine is added tothe crude (meth)acrylic acid prior to feeding to the distillationcolumn, and the crude (meth)acrylic acid having the hydrazine addedthereto is heated to a temperature lower than 80° C. and then fed to thedistillation column.

(22) A method for purifying (meth)acrylic acid, which comprisesdistilling and purifying acrylic acid or methacrylic acid obtained by avapor phase catalytic oxidation method (hereinafter referred to as acrude (meth)acrylic acid), characterized in that the distillation iscarried out at a bottom temperature of not higher than 110° C. in thepresence of a polymerization inhibitor comprising copper (meth)acrylateand/or copper dithiocarbamate, and a hydrazine compound.

(23) The method according to the above (22), wherein the copper(meth)acrylate is mixed to the crude (meth)acrylic acid and/or the topliquid.

(24) The method according to the above (22) or (23), wherein the copperdithiocarbamate is mixed to the crude (meth)acrylic acid and/or the topliquid.

(25) The method according to any one of the above (22) to (24), whereinthe copper (meth)acrylate is a solution obtained by dissolving at leastone compound selected from copper powder, cupric carbonate, cuproushydroxide, cupric hydroxide and copper acetate, in acrylic acid.

(26) The method according to any one of the above (22) to (25), whereinthe copper dithiocarbamate is copper dimethyldithiocarbamate, copperdiethyldithiocarbamate, copper dipropyldithiocarbamate, copperdibutyldithiocarbamate, copper ethylenedithiocarbamate, coppertetramethylenedithiocarbamate, copper pentamethylenedithiocarbamate,copper hexamethylenedithiocarbamate or copperoxydiethylenedithiocarbamate.

(27) The method according to any one of the above (22) to (26), whereinthe hydrazine compound is hydrazine, hydrazine hydrate, phenylhydrazine, hydrazine sulfate or hydrazine hydrochloride.

(28) The method according to any one of the above (22) to (27), whereinthe crude (meth)acrylic acid is distilled in the presence of a phenolcompound.

(29) The method according to any one of the above (22) to (28), whereinthe crude (meth)acrylic acid is distilled in the presence of aphenothiazine compound.

(30) The method according to any one of the above (22) to (29), whereinthe distillation is carried out continuously by maintaining thetemperature at a bottom temperature of at least 80° C.

The present invention has the following preferred embodiments (a) to(f).

a1. A method for purifying a (meth)acrylic acid obtained by a vaporphase catalytic oxidation method, characterized in that a crude(meth)acrylic acid having impurities tentatively removed via apreliminary purification step, and a top fraction from a thirddistillation column, are fed to and distilled in a first distillationcolumn of a purification system comprising first to third threedistillation columns, the top fraction from the first distillationcolumn is, after applying an aldehyde removal treatment thereto or afteradding an aldehyde removing agent, fed to and distilled in the seconddistillation column, the resulting top fraction is recovered as aproduct, the bottoms from the first and second distillation columns arefed to and distilled in the third distillation column, the resulting topfraction is fed to the first distillation column, and the bottomfraction is discharged out of the purification system.

a2. The method according to a1, wherein the bottoms from the thirddistillation column are decomposed by subjecting them to a thermaldecomposition apparatus, and a low boiling point component containingformed (meth)acrylic acid, is supplied to a preliminary purificationstep, while a heavy component is discharged out of the system.

a3. The method according to a1 or a2, wherein as the third distillationcolumn, a thin film evaporator is used.

a4. The method according to any one of a1 to a3, wherein the crude(meth)acrylic acid supplied to the first distillation column contains atleast 85 wt % of (meth)acrylic acid, and the rest is a higher boilingpoint component than (meth)acrylic acid.

b1. A method for producing (meth)acrylic acid, which comprises feeding acrude (meth)acrylic acid obtained by vapor phase catalytic oxidation toa distillation column to continuously distil and purify it in thepresence of a hydrazine, characterized in that the hydrazine is added tothe crude (meth)acrylic acid prior to feeding to the distillationcolumn, and the crude (meth)acrylic acid having the hydrazine addedthereto is heated to a temperature lower than 80° C. and then fed to thedistillation column.

b2. The method according to b1, wherein the crude (meth)acrylic acidhaving the hydrazine added thereto is heated to a temperature of atleast 60° C. and less than 80° C. and then fed to the distillationcolumn.

c1. A method for purifying (meth)acrylic acid, which comprisesdistilling and purifying acrylic acid or methacrylic acid obtained by avapor phase catalytic oxidation method (hereinafter referred to as acrude (meth)acrylic acid), characterized in that the distillation iscarried out at a bottom temperature of not higher than 110° C. in thepresence of a polymerization inhibitor comprising copper (meth)acrylateand/or copper dithiocarbamate, and a hydrazine compound.

c2. The method according to c1, wherein a packed column, a perforatedplate column or a distillation column consisting of a combinationthereof, is used, and continuous distillation is carried out whilemaintaining the bottom temperature to a level of at most 110° C.

c3. The method according to c2, wherein the copper (meth)acrylate ismixed to the crude (meth)acrylic acid and/or the top liquid.

c4. The method according to c2 or c3, wherein the copper dithiocarbamateis mixed to the crude (meth)acrylic acid and/or the top liquid.

c5. The method according to any one of c1 to c4, wherein the copper(meth)acrylate is a solution obtained by dissolving at least onecompound selected from copper powder, cupric carbonate, cuproushydroxide, cupric hydroxide and copper acetate, in acrylic acid.

c6. The method according to any one of c1 to c5, wherein the copperdithiocarbamate is copper dimethyldithiocarbamate, copperdiethyldithiocarbamate, copper dipropyldithiocarbamate, copperdibutyldithiocarbamate, copper ethylenedithiocarbamate, coppertetramethylenedithiocarbamate, copper pentamethylenedithiocarbamate,copper hexamethylenedithiocarbamate or copperoxydiethylenedithiocarbamate.

c7. The method according to any one of c1 to c6, wherein the hydrazinecompound is hydrazine, hydrazine hydrate, phenyl hydrazine, hydrazinesulfate or hydrazine hydrochloride.

c8. The method according to any one of c1 to c7, wherein the crude(meth)acrylic acid is distilled in the presence of a phenol compound.

c9. The method according to any one of c1 to c8, wherein the crude(meth)acrylic acid is distilled in the presence of a phenothiazinecompound.

c10. The method according to any one of c1 to c9, wherein thedistillation is carried out continuously by maintaining the temperatureat a bottom temperature of at least 80° C.

d1. A thin film evaporator being a vertical thin film evaporator whichcomprises an evaporator main body with its principal portion beingcylindrical, which has a heating means on its exterior surface, a liquidinlet and a vapor outlet at its upper portion and a residue dischargeport at its lower portion, a rotary shaft set therein, and stirringvanes attached to the shaft and being movable in a peripheral directionalong the inner wall surface of the evaporator main body, characterizedin that it has wipers movable in a peripheral direction in contact withthe inner wall surface between the lower end of the stirring vanes andthe residue discharge port.

d2. A thin film evaporator being a vertical thin film evaporator whichcomprises an evaporator main body with its principal portion beingcylindrical and its lower portion constituting an inverted cone-shapedliquid collection portion, which has a heating means on its exteriorsurface, a liquid inlet and a vapor outlet at its upper portion and aresidue discharge port at its lower portion, a rotary shaft set in themain body, and stirring vanes attached to the shaft and being movable ina peripheral direction along the inner wall surface of the evaporatormain body, characterized in that it has wipers movable in a peripheraldirection in contact with the inner wall surface of the invertedcone-shaped liquid collection portion.

d3. A thin film evaporator being a vertical thin film evaporator whichcomprises an evaporator main body with its principal portion beingcylindrical and its lower portion constituting a funnel-shaped liquidcollection portion with a combination of an inverted cone-shape and acylindrical shape, which has a heating means on its exterior surface, aliquid inlet and a vapor outlet at its upper portion and a residuedischarge port at its lower portion, a rotary shaft set in the mainbody, and stirring vanes attached to the shaft and being movable in aperipheral direction along the inner wall surface of the evaporator mainbody, characterized in that it has wipers movable in a peripheraldirection in contact with the inner wall surface of the funnel-shapedliquid collection portion.

d4. A thin film evaporator being a vertical thin film evaporator whichcomprises an evaporator main body with its principal portion beingcylindrical, which has a heating means on its exterior surface while thecorresponding interior surface constitutes a heat transfer surface, andhas a liquid inlet and a vapor outlet at its upper portion and a residuedischarge port at its lower portion, a rotary shaft set in the mainbody, and stirring vanes attached to the shaft and being movable in aperipheral direction along the inner wall surface of the evaporator mainbody, characterized in that it has wipers movable in a peripheraldirection in contact with the inner wall surface being a non-heattransfer surface at the lower portion of the evaporator main body.

d5. The thin film evaporator according to any one of d1 to d4, whereinthe wipers are attached to the same rotary shaft as the rotary shaft towhich the stirring vanes are attached.

d6. The thin film evaporator according to any one of d1 to d5, whereinthe wipers are of a movable vane type.

d7. A method for separating a liquid comprising a readily polymerizablecomponent into a vapor and an evaporation residue, characterized in thatinto the thin film evaporator as defined in any one of d1 to d6, aliquid containing a readily polymerizable component, is supplied fromthe upper liquid inlet and permitted to flow on an inner wall surface, avapor generated is withdrawn from the upper vapor outlet to theexterior, and the evaporation residue is withdrawn from the lowerresidue discharge port to the exterior.

d8. A method for recovering (meth)acrylic acid or its ester from adistillation residue of (meth)acrylic acid or its ester, characterizedby feeding a liquid containing (meth)acrylic acid or its ester to thethin film evaporator as defined in any one of d1 to d6 from the liquidinlet at its upper portion to let it flow down on the inner wallsurface, wherein a vapor of the (meth)acrylic acid or its ester formed,is withdrawn from the vapor outlet at its upper portion to outside, andthe distillation residue is withdrawn from the residue discharge port tooutside.

e1. A method for producing an acrylic ester, which comprises reacting anacrylic acid with an alcohol, wherein as the acrylic acid, an acrylicacid is used which contains at most 1,000 weight ppm ofβ-acryloxypropionic acid, at most 500 weight ppm in a total amount offurfural and benzaldehyde, and at most 2,000 weight ppm of maleicanhydride.

e2. The method according to e1, wherein the content ofβ-acryloxypropionic acid is at most 500 weight ppm.

e3. The method according to e1 or e2, wherein the acrylic acid is anacrylic acid obtained by a vapor phase catalytic oxidation reaction ofpropylene.

f1. A method for producing a high purity (meth)acrylic acid, whichcomprises extracting and/or distilling a reaction product containing a(meth)acrylic acid obtained by vapor phase catalytic oxidation to removelow boiling point impurities and high boiling point impurities from thereaction product thereby to obtain a purified (meth)acrylic acid,treating the purified (meth)acrylic acid with an aldehyde removingagent, and then distilling it in a distillation column to obtain a highpurity (meth)acrylic acid from the top of the distillation column,characterized in that the bottom fraction from the distillation columnis used as a material for producing a (meth)acrylic ester.

f2. The method according to f1, wherein the purified (meth)acrylic acidcontains aldehydes having boiling points close to (meth)acrylic acid.

f3. The method according to f1 or f2, wherein the (meth)acrylic ester ismethyl (meth)acrylate and/or ethyl (meth)acrylate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of a flow sheet to put the present invention inpractice.

FIG. 2 is a schematic view of an embodiment of the thin film evaporatoraccording to the present invention.

FIG. 3 is an enlarged view of the bottom portion of the thin filmevaporator in FIG. 2.

FIG. 4 is a view showing the state in which the wipers are attached tocorrespond to the inverted corn-shaped portion in FIG. 3.

FIG. 5 is a view showing the state in which the wipers are attached tocorrespond to the cylindrical portion in FIG. 3.

FIG. 6 is a flow chart showing an embodiment of the process forproducing acrylic acid, to which the method for producing high purity(meth)acrylic acid of the present invention can be applied.

In the drawings, reference numeral 1 indicates a first distillationcolumn, 2 a second distillation column, 3 a third distillation column,4, an ion exchange resin column, 5 a thermal decomposition column, 6 asupply tube for crude acrylic acid, 7 a supply tube for analdehyde-removing agent, 8 a withdrawing tube for purified acrylic acid(for ester), 9 a withdrawing tube for purified acrylic acid (for ahighly water-absorptive resin), 10 a withdrawing tube for thermaldecomposition fraction, 11 a withdrawing tube for thermal decompositionresidue, 21 a heating jacket, 22 a principal portion, 23 a rotationalshaft, 24 a motor, 25 a stirring vane, 26 a liquid inlet, 27 a vaporoutlet, 28 a residue discharge port, 29 a liquid collection portion, 30a wiper, 31 a wiper, 32 a wiper supporting arm, 33 a spring, 41 anacrylic acid collection column, 42 a distillation column fordehydration, 43 a decanter, 44 a distillation column for separating alow boiling point component, 44A, 45A, 47A reflux tanks, 45 adistillation column for separating high boiling point component, 46 areactor to change aldehyde to a heavy substance, and 47 a distillationcolumn for purification.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment a

In the present invention, the vapor phase catalytic oxidation and thesubsequent preliminary purification may be carried out by conventionalmethods. For example, in the case of acrylic acid, a method forobtaining acrylic acid by a one step oxidation method of propane bymeans of a Mo—V—Te double oxide catalyst or a Mo—V—Sb double oxidecatalyst, or a one step oxidation method of oxidizing propylene directlyto acrylic acid, and a two step oxidation method of converting propyleneto acrolein and then oxidizing acrolein to acrylic acid, are known, andany one of the methods may be employed. Further, acrylic acid formed bythe vapor phase catalytic oxidation is usually absorbed in water to forman aqueous acrylic acid solution, and recovery of crude acrylic acidfrom this aqueous acrylic acid solution may also be carried out by aconventional method. For example, a method may be employed wherein afterdehydration by azeotropic distillation, distillation is further carriedout to remove acetic acid and other low boiling point components. Thepurity of crude (meth)acrylic acid thus obtained is usually at least 85wt %, in many cases at least 90 wt %. As a matter of course, the higherthe purity of this crude (meth)acrylic acid, the better. Impuritiescontained in this crude (meth)acrylic acid are a dimer of (meth)acrylicacid and other heavy components, and low boiling point components arenot substantially contained.

In the present invention, such crude (meth)acrylic acid is distilled andpurified by a purification system comprising first to third threedistillation columns, to recover high purity (meth)acrylic acid suitablefor application to a highly water absorptive resin. Firstly, the crudeacrylic acid and the top fraction from the third distillation column arefed to and distilled in the first distillation column. The ratio of thetwo fed to the first distillation column varies depending upon theoperation conditions of the purification system, particularly on howmuch of the top fraction from the first distillation column will be fedto the second distillation column. As the first distillation column, itis common to use one having a theoretical plate number of from 5 to 20plates, and it is operated under reduced pressure, whereby (meth)acrylicacid is distilled from the top of the column. This (meth)acrylic acidusually has a purity of at least 99.5 wt %, in many cases at least 99.7wt %, and thus has a sufficient purity for a (meth)acrylic ester.However, it still contains an aldehyde component such as furfural orbenzaldehyde, and as such, is not adequate as a starting material for ahighly water absorptive resin.

In a preferred embodiment of the present invention, the top fractionfrom the first distillation column is treated with an aldehyde-removingagent, or it is fed together with an aldehyde-removing agent to thesecond distillation column, followed by distillation. As disclosed inJP-A-2001-58970 or JP-A-2001-213839, it is known to remove an aldehydecomponent by treating (meth)acrylic acid containing the aldehydecomponent with an aldehyde-removing agent. As the aldehyde-removingagent, in addition to a primary amine or a hydrazine disclosed in thesepublications, a mercaptan such as n-butylmercaptan, n-octylmercaptan orn-dodecylmercaptan may, for example, be used. In such a case, afteradding such a mercaptan, the top fraction is then treated with asulfonic acid type cation exchange resin. Removal of the aldehydecomponent by the primary amine or the hydrazine may be carried outbefore feeding the top fraction from the first distillation column tothe second distillation column, or the aldehyde-removing agent may besupplied to the second distillation column together with or separatelyfrom the top fraction, so that the aldehyde removal reaction is carriedout in the column. Further, in a case where a mercaptan is to be used,one having a mercaptan added to the top fraction from the firstdistillation column is passed through a resin column packed with asulfonic acid type cation exchange resin at a temperature of from 20 to90° C. at SV=0.1 to 10 hr⁻¹ to carry out removal of the aldehydecomponent. Passing of the liquid may be a down flow system or an up flowsystem. The aldehyde-removing agent is used usually in an amount of from1 to 8 times by mol relative to the aldehyde component.

As the second distillation column, it is common to employ one having atheoretical plate number of from 1 to 5 plates, and it is operated underreduced pressure, and (meth)acrylic acid is distilled from the top ofthe column. For example, in the case of acrylic acid, it is preferred toadjust such that the bottoms will be from 50 to 100° C., and theretention time in the column will be from about 1 to 2 hours. Further,the concentration rate of the bottoms, i.e. the weight ratio of the topfraction from the first distillation column to be fed, to the liquidpermitted to flow out from the bottom of the column, is preferably from2 to 25. The top fraction from the second distillation column is ofextremely high purity, usually at least 99.8 wt %, in many cases atleast 99.9 wt %, and contains no aldehydes, whereby it is suitable asthe starting material for a highly water absorptive resin.

The bottoms from the first and second distillation columns still containa large amount of (meth)acrylic acid, and therefore, they are fed to anddistilled in the third distillation column, and from the top thereof,(meth)acrylic acid is distilled and supplied to the first distillationcolumn, whereby the amount of (meth)acrylic acid discharged out of thepurification system can be reduced, and the recovery rate of(meth)acrylic acid can be improved. The top fraction from the thirddistillation column is further distilled in the first distillationcolumn, whereby even if a heavy component other than (meth)acrylic acidis contained as accompanied by splash in the top fraction, such will notbe problematic.

As the third distillation column, it is preferred to employ a thin filmevaporating apparatus. It is well is known that for this apparatus,there are a vertical type and a horizontal type. Typically, in eithertype, in the interior of a cylinder having a jacket, rotary stirringvanes or wipers are installed, so that a thin film of a supplied liquidis formed on the inner surface of the cylinder so as to be evaporated.It is particularly preferred to employ a vertical type such as a Smithtype thin film evaporator or a Luwa type thin film evaporator. Also thethird distillation column is preferably operated under reduced pressure,for example, in the case of acrylic acid, under a pressure at a level offrom 67 Pa to 40 KPa. It is thereby possible to lower the operationtemperature and thereby to suppress polymerization, etc. of(meth)acrylic acid.

In a preferred embodiment of the present invention, the bottoms from thethird distillation column are fed to and thermally decomposed in athermal decomposition apparatus. Such bottoms comprise non-evaporated(meth)acrylic acid, its dimer, the aldehyde-removing agent, maleic acidsand other impurities, and accordingly, (meth)acrylic acid can berecovered by this thermal decomposition. In the purification bydistillation of (meth)acrylic acid, it is known to thermally decomposethe bottoms to recover (meth)acrylic acid, and also in the presentinvention, the recovery may be carried out in accordance with such aknown method. For example, the temperature is usually preferably from110 to 250° C., particularly preferably from 120 to 230° C., and thetime required for the decomposition is, in the case of a lowtemperature, usually from 10 to 50 hours, and in the case of a hightemperature, from 0.5 to 10 hours. The pressure may be atmosphericpressure or reduced pressure. A low boiling fraction containing(meth)acrylic acid obtained by thermal decomposition contains lowboiling point components, etc., and therefore, it is supplied to a stageprior to the stage for removal of low boiling point components in thepreliminary purification step. The heavy component is discharged out ofthe system and incinerated.

Embodiment b

In order to solve the conventional problems in continuous production ofhigh purity acrylic acid on an industrial scale, the present inventorshave conducted extensive studies on the relation, etc. of thealdehyde-removing agent, various additives and their amounts, formationof sludge and its thermal stability, and the amounts of impuritiesremaining in purified acrylic acid, and as a result, have found thefollowing facts. Namely, usually, when crude acrylic acid having aconcentration of maleic acids being at least 2000 ppm, was used andreacted with a hydrazine, solid would precipitate, and if such astarting material was fed to the side of a distillation column,continuous distillation was impossible due to clogging by theprecipitated solid. Whereas, when the starting material is reacted withhydrazine prior to feeding it to the side of the distillation column,followed by heat treatment at a temperature lower than 80° C., itbecomes possible to suppress re-formation of maleic acid once removed bythe reaction with hydrazine and to have precipitated solid formed into auniform solution (namely, a state where no formation of precipitate isobserved even when the solution is left to stand for 30 minutes), and ithas been found possible to suppress formation of sludge in thedistillation column even by continuous distillation on a commercialscale. The present invention has been made based on such a finding.

Further, here, the method for purifying (meth)acrylic acid of thepresent invention, will be described with respect to acrylic acid, butthe present invention can be applied to methacrylic acid in the samemanner. When the present invention is to be applied to the production ofmethacrylic acid, crude methacrylic acid may be obtained by vapor phasecatalytic oxidation of isobutylene and/or t-butyl alcohol, and in suchcrude methacrylic acid, aldehydes, ketones, maleic acids as well ascitraconic acids, are contained as impurities, in the same manner as inthe case of crude acrylic acid.

The crude acrylic acid to be purified by the present invention is oneobtainable by vapor phase catalytic oxidation, which contains maleicacids, etc. as impurities, and it is usually produced industrially bythe following methods.

Namely, it is produced by a one step oxidation method wherein propane,propylene and/or acrolein is reacted with a molecular oxygen-containinggas in the presence of e.g. a molybdenum oxide type solid oxidizedcatalyst as a solid catalyst, or a two step oxidation method wherein inthe presence of a solid catalyst such as a molybdenum oxide type solidoxidized catalyst, firstly, in the first reaction zone, acrolein isobtained by the reaction of propylene with a molecular oxygen-containinggas, and in the subsequent second reaction zone, the acrolein is reactedwith molecular oxygen in the presence of a solid catalyst such as amolybdenum oxide solid oxidized catalyst to obtain acrylic acid. Or, bya method of obtaining an acrylic acid by oxidizing propane by means of aMo—V—Te type double oxide catalyst or a Mo—V—Sb type double oxidecatalyst, a formed gas of a vapor phase catalytic oxidation reaction, isobtained, and this formed gas is countercurrently contacted with waterin an absorption column to obtain an aqueous crude acrylic acid. Thisaqueous crude acrylic acid solution is extracted with an organic solventsuch as methyl isobutyl ketone or diisobutyl ketone, followed bydistillation, or an azeotropic agent such as toluene, butyl acetate oroctane is added thereto, followed by direct azeotropic dehydration undersuch conditions as a bottom temperature of from 80 to 100° C. and apressure of from 6.67 to 20 kPa to obtain an acrylic acid-containingliquid. From the obtained acrylic acid-containing liquid, a low boilingpoint component such as acetic acid, is removed, and the bottoms arefurther distilled to obtain crude acrylic acid as the top fraction, anda high boiling point component such as a dimer, is withdrawn from thebottom of the column.

The crude acrylic acid to be used as a starting material for high purityacrylic acid, in the present invention, is the top fraction in thedistillation step after removing such a low boiling point component, andif recovery of acrylic acid from the dimer, etc., is taken intoconsideration, this crude acrylic acid usually contains, as impurities,carboxylic acids such as maleic acids and acetic acid, aldehydes such asfurfural and benzaldehyde, water, etc.

As the crude acrylic acid to be used in the process for producing highpurity acrylic acid in the present invention, it is preferred to employone having a concentration of maleic acids being at least 2000 ppm.Further, the upper limit of the concentration of maleic acids ispreferably 10000 ppm, more preferably 5000 ppm. In order to treat onehaving maleic acids more than this, the amount of the required hydrazineincreases, such being uneconomical. Whereas, in order to employ crudeacrylic acid having a concentration of maleic acids less than 2000 ppm,it is necessary to increase the plate number of the distillation columnin order to increase the precision for separation of acrylic acid andmaleic acids in the process for producing crude acrylic acid, or it isnecessary to stop recovery of acrylic acid from a high boiling pointproduct containing a dimer of acrylic acid in order to reduce the amountof maleic acids distilled from the top of the column at the same time ascarrying out the recovery of acrylic acid from the dimer of acrylic acidand to dispose the entire amount, such being undesirable as theeconomical loss is substantial.

In the present invention, a hydrazine is added to crude acrylic acidprior to feeding to the side of the distillation column, topreliminarily react the hydrazine with maleic acids in the crude acrylicacid, and then purification by distillation is carried out. As thereaction apparatus to be used for the reaction of the hydrazine withmaleic acids in the crude acrylic acid, any one may be used so long asthe necessary temperature and the retention time can be secured. Forexample, a reaction tank equipped with a stirrer or a tubular reactiontank may be employed. The reaction temperature is preferably as low aspossible. Specifically, it is selected within a range of at least themelting point of acrylic acid and at most 50° C. As the reaction time,it is preferred to retain at least 10 minutes, usually from 30 minutesto 3 hours.

As the hydrazine to be added to the crude acrylic acid, it is preferredto add hydrazine and/or hydrazine hydrate as it is. The amount of thehydrazine is usually from 0.1 to 2 times by mol, preferably from 0.5 to2 times by mol, more preferably from 0.5 to 1 time by mol, to the totalamount of maleic acids and aldehydes such as furfural and benzaldehyde,in the crude acrylic acid.

After the above reaction, the reaction mixture of the crude acrylic acidand the hydrazine, is heated before it is fed to a distillation column.The upper limit of this heating temperature (hereinafter sometimesreferred to as “the feeding temperature”) is lower than 80° C., but apreferred upper limit is 75° C. Further, a preferred lower limit of thefeeding temperature is 60° C., but more preferred lower limit is 62° C.If the feeding temperature is lower than 60° C., solid formed by thereaction of maleic acids and the hydrazine will be precipitated andslurried, and if such a slurry is fed into a distillation column as itis, such will cause deposition or formation of sludge in thedistillation column, such being undesirable. On the other hand, if thefeeding temperature is 80° C. or higher, from the adduct once formed bythe reaction of the hydrazine and maleic acids, maleic acid will bere-produced by a reverse reaction, and such maleic acid will bedistilled from the top of the distillation column, and besides, therewill be a problem of polymerization of thermally unstable acrylic acidby heating at a high temperature, such being undesirable.

The method for heating the reaction solution of the hydrazine and thecrude acrylic acid is not particularly limited so long as the internaltemperature can be set at the above-mentioned temperature. For example,this reaction solution may be heated by means of a heat exchanger andthen fed directly to the side of a distillation column.

The heating time of this reaction solution may depend on the content ofmaleic acids in the crude acrylic acid. However, once the internaltemperature of the reaction solution reaches the prescribed temperature,the yellow precipitated solid by the reaction of maleic acids and thehydrazine, will disappear, and a uniform solution will be formed,whereby the end of the heating time can easily be ascertained.Accordingly, the time for this disappearance may be taken as the endpoint of the heating. In a usual case, one hour is sufficient for such aheating time. A heating time of more than that is not desirable, sincealdehydes removed by the reaction with the hydrazine are likely toundergo a reverse reaction.

The operation conditions of the distillation column into which thisheated reaction solution is fed, vary depending upon the composition ofthe material to be distilled, the recovery rate, the purity of acrylicacid distillate, etc. However, as acrylic acid is a readilypolymerizable compound, the distillation temperature and pressure arepreferably set so that they will be a low temperature and low pressureas far as possible. Specifically, usually, the bottom temperature isfrom 60 to 100° C., and the top pressure is selected within a range offrom 1.33 to 26.7 kPa.

In the present invention, at the time of distillation, in addition to ahydrazine as an agent for treating impurities, a conventional knownpolymerization-preventing agent i.e. a polymerization inhibitor and/or apolymerization controlling agent may be added. As such a polymerizationpreventing agent, various studies have already been made. The followingones may be mentioned as examples of the polymerization-preventingagent. Namely, an N-oxyl compound may, for example, be tertiary butylnitrooxide, 2,2,6,6-tetramethyl-4-hydroxypiperidyl-1-oxyl,2,2,6,6-tetramethylpiperidyl-1-oxyl, 2,2,6,6-tetramethylpiperidinooxyl,4-hydroxy-2,2,6,6-tetramethylpiperidinooxyl or4,4′,4″-tris1-(2,2,6,6-tetramethylpiperidinooxyl)phosphite, a phenolcompound may, for example, be hydroquinone, methoquinone, pyrogallol,catechol, or resorcinol; a phenothiazine compound may, for example, bephenothiazine, bis-(α-methylbenzyl)phenothiazine,3,7-dioctylphenothiazine, or bis(α-dimethylbenzyl)phenothiazine; and acopper compound may, for example, be cupric chloride, copper acetate,copper carbonate, copper acrylate, copper dimethyldithiocarbamate,copper diethyldithiocarbamate, or copper dibutyldithiocarbamate. Thesepolymerization-preventing agents may be used alone or in combination asa mixture of two or more of them. The amount of such apolymerization-preventing agent is not particularly limited, but ispreferably at a level of from 1 to 1000 ppm.

In the present invention, the method for distillation is notparticularly limited. For example, various methods such as simpledistillation, precision distillation, etc. may be employed. Suchdistillation may be carried out either in a batch system or in acontinuous system. However, from the industrial point of view, it ispreferred to carry out the distillation in a continuous system. Further,also with respect to the distillation apparatus, there is no particularrestriction.

As a distillation column, a perforated plate column, a bubble-capcolumn, a packed column or a combination thereof (such as a combinationof a perforated plate column and a packed column) may, for example, beavailable, and any one may be used in the present invention withoutdistinguishing the presence or absence of an overflow gate or a downcomer. As specific trays, bubble cap trays, perforated plate trays,bubble trays, super flash trays, max flux trays, or dual trays may, forexample, be mentioned.

As packing materials, in addition to those which are heretofore beenused, such as columnar, cylindrical, saddle-type, spherical, cubic orpyramid-shaped ones, regular or irregular packing materials havingspecific shapes have been commercially available as high performancepacking materials in recent years. These packing materials may suitablybe used in the present invention.

Examples of such commercial products include, as a regular packingmaterial, a gauze type regular packing material such as Sulzer Packing(manufactured by Sulzer Brothers Company), Sumitomo Sulzer Packing(manufactured by Sumitomo Heavy Industries, Ltd.) or Tecknopack(manufactured by Mitsui & Co., Ltd.), a sheet type regular packingmaterial such as Mellapack (manufactured by Sumitomo Heavy Industries,Ltd.), Tecknopack (manufactured by Mitsui & Co., Ltd.), or MC Pack(manufactured by Mitsubishi Chemical Engineering Corporation), and agrid type regular packing material such as Flexigrid (manufactured byKoch Company). Further, GEMPAK (manufactured by Glitsch Company), MontzPack (manufactured by Montz Company), Goodroll Packing (manufactured byTokyo Tokushu Kanaami K.K.), Honeycomb Pack (Manufactured by NGKInsulators, Ltd.) and Impulse Packing (Manufactured by NagaokaCorporation) may, for example, be mentioned.

Further, an irregular packing material may, for example be Raschig ring,Pall ring (manufactured by BASF), Cascade Miniring (manufactured by MassTransfer Company), IMTP (manufactured by Norton Company), Intalox Saddle(manufactured by Norton Company), Tellerette (manufactured by NittetsuChemical Engineering Ltd.) or Flexiring (manufactured by JGCCorporation).

The material for the apparatus constituting the distillation column, isnot particularly limited, but since (meth)acrylic acid is corrosive, itis preferred to use a stainless steel such as SUS304, SUS304L, SUS316,SUS316L, SUS317, SUS317L, SUS329J1 or SUS329J2L, or a nickel alloy suchas hastelloy or inconel.

Embodiment c

The (meth)acrylic acid to be used here, is one obtained by vapor phasecatalytic oxidation of propane, propylene and/or acrolein, orisobutylene and/or methacrolein. For example, it may be acrylic acidwhich is obtained by vapor phase oxidation of propane by means of aMo—V—Te double oxide catalyst or a Mo—V—Sb double oxide catalyst, oracrylic acid or methacrylic acid which is obtained by vapor phasecatalytic oxidation of propylene or isobutylene in the presence of aMo—Bi double oxide catalyst to form acrolein or methacrolein, which isfurther subjected to vapor phase catalytic oxidation in the presence ofa Mo—V double oxide catalyst. Here, the preliminary reaction ofoxidizing propylene to form mainly acrolein and the subsequent reactionof oxidizing acrolein to form mainly acrylic acid, may be carried out inseparate reactors, respectively, or the catalyst for the preliminaryreaction and the catalyst for the subsequent reaction may simultaneouslybe packed into one reactor to carry out the reactions.

Such crude (meth)acrylic acid is one containing impurities formed asby-products in the production process. Such impurities may, for example,be low boiling point impurities such as water, furfural, benzaldehyde,acetic acid, etc., and high boiling point impurities such as a dimer ortrimer of (meth)acrylic acid, maleic anhydride, β-hydroxypropionic acid,β-alkoxypropionic acid, etc.

The hydrazine compound to be used in the present invention is one whichacts to convert a compound having a boiling point close to (meth)acrylicacid, such as furfural, to a component which can be easily distilled andseparated.

Such a hydrazine compound may, for example, be hydrazine, hydratedhydrazine, phenyl hydrazine, hydrazine sulfate or hydrazinehydrochloride. They may be used alone or in combination as a mixture oftwo or more of them. The amount of the hydrazine compound to be added,is suitably selected depending upon the amount of impurities to beremoved and the concentration of impurities allowed to be contained inhigh purity acrylic acid obtainable after the distillation.

In the present invention, it is used usually in an amount of from 1 to10 times by weight, preferably from 2 to 5 times by weight, based on theweight of impurities to be removed, contained in the starting material(meth)acrylic acid. It is used usually in an amount of from 50 to 5000ppm, preferably from 200 to 4000 ppm, as represented based on the crude(meth)acrylic acid. If its amount is small, impurities to be removed,will be contained in a large amount in purified (meth)acrylic acid, suchbeing undesirable. If the amount is large, such will not be problematicfor the removal of impurities, but the effect by addition will besaturated, and such is not economically desired.

The copper dithiocarbamate to be used in the present invention is onewhich acts as a polymerization inhibitor (a polymerization-preventingagent) for (meth)acrylic acid.

Such a copper dithiocarbamate may, for example, be a copperdialkyldithiocarbamate such as copper dimethyldithiocarbamate, copperdiethyldithiocarbamate, copper dipropyldithiocarbamate or copperdibutyldithiocarbamate, a copper cyclic alkylene dithiocarbamate such ascopper ethylenedithiocarbamate, copper tetramethylenedithiocarbamate,copper pentamethylenedithiocarbamate or copperhexamethylenedithiocarbamate, or a copper cyclicoxydialkylenedithiocarbamate such as copperoxydiethylenedithiocarbamate. They may be used alone or in combinationas a mixture of one or more of them.

The amount of the copper dithiocarbamate is from 1 to 100 weight ppm,preferably from 10 to 80 weight ppm, based on the (meth)acrylic acid tobe fed to the distillation column. If the amount is small, the effectfor suppressing polymerization tends to be inadequate. If the amount islarge, corrosion of the apparatus at the bottom of the distillationcolumn tends to take place, such being undesirable. It is consideredthat in the distillation system of the present invention, the copperdithiocarbamate has a larger effect for suppressing polymerization ofthe bottoms than suppression of the polymerization of the liquid whichflows down the interior of the distillation column. Accordingly, withrespect to the position for the addition of the copper dithiocarbamate,it is preferred to add it to the crude (meth)acrylic acid as thestarting material, or to the bottoms of the distillation column.

Copper (meth)acrylate to be used in the present invention acts as apolymerization inhibitor (a polymerization preventing agent) for(meth)acrylic acid, like the copper dithiocarbamate. By a combined useof the two, a remarkable effect can be obtained for the first time. Thecopper (meth)acrylate can be prepared by dissolving a carbonate, achloride, an organic salt or a hydroxide of copper, or a copper powderin (meth)acrylic acid. Copper carbonate is particularly preferred. Thechloride is not preferred, since stress corrosion cracking is likely totake place, as the distillation column for (meth)acrylic acid, isusually made of a stainless steel material. With respect to specificmaterials to be dissolved in (meth)acrylic acid in order to obtaincopper (meth)acrylate to be used in the present invention, the carbonatemay, for example, be cupric carbonate; the salt of an organic acid may,for example, be copper formate, copper acetate or copper salicylate; andthe hydroxide may, for example, be cuprous hydroxide or cuprichydroxide. Further, copper powder may directly be dissolved in(meth)acrylic acid. They may be used alone or in combination as amixture of two or more of them.

The copper (meth)acrylate may be obtained also by dissolving such amaterial in a solvent containing (meth)acrylic acid. As the solvent insuch a case, it is preferred to employ a solvent having a boiling pointhigher than (meth)acrylic acid, so that the solvent will not be includedin high purity (meth)acrylic acid obtained from the top of thedistillation column. Specifically, diphenyl ether, an o-phthalic acidester, an oleic acid ester, an adipic acid ester, a hydrocarbon in amedium oil fraction, a heat conductive oil having a boiling point of atleast 170° C., or a mixed solvent thereof, may be used.

In a case where water is contained in the crude (meth)acrylic acid asthe starting material to be distilled, water may also be used as thesolvent having a boiling point lower than (meth)acrylic acid. Theconcentration of water may be determined taking into consideration thevalue allowable for high purity (meth)acrylic acid to be obtained andthe necessary amount of the copper (meth)acrylate. In a case where nowater is contained in the crude (meth)acrylic acid, a due care will berequired, since dehydration may again be required depending upon thespecification for the product.

The amount of the copper (meth)acrylate may be calculated on theassumption that the copper dissolved is all converted to copper(meth)acrylate, and it is from 1 to 100 weight ppm, preferably from 5 to80 weight ppm, based on the crude (meth)acrylic acid to be fed to thedistillation column. If the amount is small, the effect for suppressingpolymerization tends to be inadequate. If the amount is large, corrosionof the apparatus at the bottom of the distillation column is likely totake place, such being undesirable.

As is different from the copper dithiocarbamate, the copper(meth)acrylate provides a substantial effect to the liquid in such astate that it flows down in the interior of the distillation column.Accordingly, with respect to the position for addition of the copper(meth)acrylate, it is preferred to add it to the crude (meth)acrylicacid as the starting material, or to the liquid at the top of thedistillation column.

In the present invention, as mentioned above, two types ofpolymerization inhibitors having different actions, are used. Even ifdissolved in (meth)acrylic acid, the copper dithiocarbamate can hardlybe converted to copper (meth)acrylate as a feature of the presentinvention, and accordingly, the latter is required to be added afresh asin the present invention.

Further, in the present invention, it is preferred to add a phenolcompound and/or a phenothiazine compound in addition to the hydrazinecompound, the copper dithiocarbamate and the copper (meth)acrylate,whereby the effects of the present invention can be further improved. Ifnecessary, in some cases, an N-oxyl compound such as tertiary butylnitroxide, 2,2,6,6-tetramethyl-4-hydroxypiperidyl-1-oxyl,2,2,6,6-tetramethylpiperidyl-1-oxyl, 2,2,6,6-tetramethylpiperidinooxyl,4-hydroxy-2,2,6,6-tetramethylpiperidinooxyl, or4,4′,4″-tris-(2,2,6,6-tetramethylpiperidinooxyl)phosphite; a phenylenediamine such as p-phenylene diamine; a nitroso compound such asN-nitrosodiphenylamine; a urea such as urea; and a thiourea such asthiourea, may be used in combination.

The phenol compound may, for example, be hydroquinone, methoquinone(methoxyhydroquinone), pyrogallol, catechol, resorcinol, phenol orcresol, and such may be used alone or in combination as a mixture of twoor more of them. The amount of the phenol compound is from 0 to 800weight ppm, preferably from 50 to 600 weight ppm, based on the crude(meth)acrylic acid to be fed to the distillation column. If the amountis small, the effect for controlling polymerization may sometimes beinadequate. If the amount is too much, such being economicallyundesirable, although there will be no adverse effect to the effect forsuppressing polymerization.

The phenothiazine compound may, for example, be phenothiazine,bis-(α-methylbenzyl)phenothiazine, 3,7-dioctylphenothiazine orbis-(α-dimethylbenzyl)phenothiazine, and such may be used alone or incombination as a mixture of two or more of them. The amount of thephenothiazine compound is from 0 to 400 weight ppm, preferably from 50to 300 weight ppm, based on the (meth)acrylic acid to be fed to thedistillation column. If the amount is small, the effect for suppressingpolymerization may sometimes be inadequate. If the amount is too large,such being economically undesirable, although there may be no adverseeffect to the effect for suppressing polymerization.

The method for adding the copper (meth)acrylate, the copperdithiocarbamate, the phenol compound and the phenothiazine compound,which are effective for suppressing polymerization, is not particularlylimited. For example, there may be a method wherein they are,respectively, directly added to the (meth)acrylic acid to be fed to thedistillation column or to the (meth)acrylic acid liquid refluxed as adistillate liquid, or a method wherein they are dissolved and added bymeans of a suitable solvent. The temperature for the addition may alsobe suitably determined.

The hydrazine compound to remove impurities and the method of itsaddition are also not particularly limited. However, the hydrazinecompound is required to react with impurities to be removed, and aretention time of preferably from 10 minutes to 5 hours, more preferablyfrom 20 minutes to 3 hours, is preferred after adding the hydrazinecompound to the crude (meth)acrylic acid until purified (meth)acrylicacid will be obtained as a distillate from the top of the distillationcolumn. If the time for the reaction is short, impurities will not besufficiently reacted. If the time for the reaction is too long,impurities may increase by a decomposition reaction of the reactedproducts. Accordingly, the time is selected within the above range.

The crude (meth)acrylic acid having the hydrazine compound, the copperacrylate and the copper dithiocarbamate added thereto, is subjected todistillation treatment, whereby impurities to be removed, will beremoved. The distillation method is not particularly limited, andvarious distillation methods such as simple distillation, precisiondistillation, etc. may be employed. Further, the distillation may beeither in a continuous system or in a batch system. However, anembodiment whereby the effects of the present invention can mostremarkably be obtained, is such that a constant operation for a longperiod of time can be accomplished in an industrial and economicalcontinuous distillation.

The distillation column to be used here, the type of the packingmaterial to be packed into the packing column, the material for theapparatus constituting the distillation column, etc. are as described inthe foregoing. With respect to the distillation temperature, the bottomtemperature is at most 110° C., preferably at most 100° C., in order toimprove the effect for suppressing polymerization by the presentinvention. Conventional distillation of (meth)acrylic acid used to becarried out at a temperature of at most 100° C., particularly at most70° C. (e.g. JP-A-7-228548k) in many cases. Whereas, according to thepresent invention, suppression of polymerization can remarkably be made,whereby the operation range of the bottom temperature can be increased.Accordingly, it is possible to carry out the operation at a bottomtemperature of preferably from 80 to 110° C., particularly preferablyfrom 90 to 105° C. The economical effect due to a reduction of the heattransfer area of the reboiler for distillation column, is extremelylarge.

Embodiment d

The thin film evaporator of the present invention to be used for theabove-mentioned third distillation column, is characterized in that ithas wipers movable in a peripheral direction in contact with the innerwall surface also at an inner wall surface portion further lower thanthe lower end of the stirring vanes. It is thereby possible to preventformation of a deposition on the inner wall surface further lower thanthe lower end of the stirring vanes of the thin film evaporator and tooperate the thin film evaporator constantly over a long period of time.

Like a known thin film evaporator, the thin film evaporator of thepresent invention comprises an evaporator main body with its principalportion being cylindrical, which has a heating means on its exteriorsurface, a liquid inlet and a vapor outlet at its upper portion and aresidue discharge port at its lower portion, a rotary shaft set in themain body, and stirring vanes attached to the shaft and being movable ina peripheral direction along the inner wall surface of the evaporatormain body. As a typical example of such a thin film evaporator, a Smithsystem thin film evaporator or a Luwa thin film evaporator may, forexample, be mentioned. The shape of a liquid collection portion at alower portion of the main body of the thin film evaporator is consideredto be preferably such a shape that it has an inclination to theevaporation surface so that the evaporation residue will smoothly flowinto the liquid withdrawal tube from the residue discharge port, and onewherein the shape of the liquid collection portion is an invertedcorn-shape or a funnel shape being a combination of an invertedcorn-shape and a cylindrical shape, is practically used.

The thin film evaporator according to the present invention ischaracterized in that in such a known thin film evaporator, wipers areprovided which are movable in a peripheral direction in contact with aninner wall surface located further lower than the inner wall surfacecorresponding to the lower end of the stirring vanes attached to therotary shaft. Stirring vanes are to accelerate evaporation from theliquid film and accordingly usually located at a position correspondingto the heat transfer surface, whereby the lower end of the stirringvanes is substantially the same position as the lower end of the heattransfer surface.

Thus, in the present invention, the wipers are provided to correspond toa non-heat transfer surface below the heat transfer surface. Forexample, in a case where the lower portion of the cylindrical main bodyof the thin film evaporator constitutes a non-heat transfer surface, thewipers are provided to correspond to such a portion. In a case where aliquid collection portion of an inverted-corn shape or a funnel shapebeing a combination of an inverted corn shape and a cylindrical shape,is connected to the cylindrical main body of the thin film evaporator,the wipers are provided to correspond to such a liquid collectionportion. The wiper to be provided may be one or may be divided into aplurality. It is usually preferred to provide the wiper(s) to correspondto the entire surface of such a portion. However, so long as the purposeof the wiper(s) to prevent deposition of an evaporation residue on thenon-heat transfer surface, it is not necessary to provide the wiper(s)to cover the entire surface. The wipers are usually attached to therotary shaft to which the stirring vanes are attached to form thin filmor to a shaft extended downwardly from the rotary shaft. It is therebypossible to have a common driving source for the stirring vanes and thewipers, and the structure of the apparatus can be simplified. Further,the attaching method of the wipers is optional, but it is preferred toattach them on the rotary shaft via fulcrums or springs in the samemanner as for the stirring vanes and to attach them in a movable vanesystem so that they are movable in a peripheral direction about therotary shaft.

As the material to constitute the wipers, a material suitable for thephysical properties of the liquid to be treated by the thin filmevaporator, may be selected for use. For example, in a case where theliquid to be treated is a highly corrosive liquid such as acrylic acid,a stainless steel such as SUS304, SUS316, SUS316L, SUS317, SUS317L,SUS329JL or SUS329J2L, or a nickel alloy such as hastelloy or inconel,may, for example, be mentioned. However, from the viewpoint of corrosionresistance and economical efficiency, SUS304, SUS316 or SUS316L ispreferred. Further, as the material of a portion of the wipers, whichwill be physically in contact with the inner surface of the thin filmevaporator, it is desired to employ a material which will not damage theinner wall surface of the thin film evaporator. Preferably, one which ishighly corrosion resistant and will not damage the inner wall surface ofthe thin film evaporator, such as Teflon, or a hybrid carbon having aresin or metal vacuum-press impregnated to carbon to have the mechanicalstrength and anti-sealing property improved and being useful even undera high temperature condition, such as Sliding Composite Carbon NC-07Emanufactured by Nippon Carbon Co., Ltd., is used. Particularly preferredis the hybrid carbon which is excellent in the dimensional stability fora long period of time and which is useful even under a high temperaturecondition.

Like a conventional thin film evaporator, the thin film evaporator ofthe present invention can be used to evaporate low boiling pointcomponents from various liquids to be treated, but is particularlysuitable for evaporating low boiling point components from a liquid tobe treated, which contains components readily polymerizable by heat. Assuch a liquid to be treated, a heavy component may be mentioned which isdischarged from the bottom of the column at the time of purification bydistillation of (meth)acrylic acid or its ester and wherein the(meth)acrylic acid or its ester still remains. An example of the(meth)acrylic ester, methyl, ethyl, butyl, isobutyl, tertiary butyl,2-ethylhexyl, 2-hydroxyethyl, 2-hydroxypropyl or methoxyethyl may, forexample, be mentioned.

Further, at the time of purification by distillation of such apolymerizable one, it is common to add a polymerization preventing agentto the liquid. For example, at the time of purification by distillationof acrylic acid, it is common to employ a polymerization preventingagent of a phenol type, such as hydroquinone or hydroquinonemonomethylether, an organic substance such as phenothiazine or N-oxyl compound, ora copper salt such as copper dialkyldithiocarbamate, copper acrylate orcopper acetate. Accordingly, in the bottoms discharged from the bottomof the distillation column, such a polymerization inhibitor isconcentrated. If the bottoms in which the polymerization preventingagent is concentrated, is treated by a thin film evaporator, thepolymerization preventing agent will be further concentrated andprecipitated, whereupon it may deposit on the wall surface, or thefluidity of the distillation residue tends to be deteriorated. By ausual thin film evaporator, smooth treatment is difficult, while by thethin film evaporator of the present invention, such can be easilytreated.

FIGS. 2 to 5 show schematic views of one embodiment of a rotary slidertype thin film evaporator according to the present invention, but thepresent invention is by no means restricted to such an embodiment.

This apparatus is a thin film evaporator having a cylindrical main bodyportion (22) having a heating jacket (21) on its exterior surface, andit has an interior rotary shaft (23) and a motor (24) to rotate theshaft. Stirring vanes (25) are attached to the rotary shaft (23) and thestirring vanes will rotate while maintaining a slight distance from theinner wall of the main body portion, and the liquid to be treated, fedfrom a liquid inlet (26) at an upper portion of the thin film evaporatorwill flow down by gravity along the inner wall of the main body portionwhile being spread in a film form by the rotating stirring vanes. In theprocess of this flowing down, a low boiling point component in theliquid to be treated will be evaporated by heat from the heating jacket.The evaporated low boiling point component will be led out of the systemfrom a vapor outlet (27) located at an upper portion of the thin filmevaporator, and the distillation residue having the majority of such alow boiling point component removed to have poor fluidity, will be ledto a liquid collection portion (29). And, wipers (30) and (31) attachedto the rotary shaft will rotate in contact with the wall surface of theliquid collecting portion, whereby the distillation residue flowing downto this liquid collection portion will constantly be removed from theinner wall surface and withdrawn from the residue discharge port (28)located at a lower portion of the thin film evaporator. It is therebypossible to prevent clogging by the evaporation residue at the outletportion of the liquid collection portion or the subsequent liquidwithdrawal tube, whereby safe operation of the thin film evaporator fora long period of time will be made possible.

An example will be shown in which a liquid to be treated, containing areadily polymerizable component, was treated by means of the thin filmevaporator of the present invention. Using the thin film evaporator ofthe present invention having wipers at positions corresponding to theinversed corn-shaped portion and the cylindrical portion, respectively,of the liquid collection portion, as shown in FIGS. 2 to 5, an operationwas carried out to recover acrylic acid from the bottoms (acrylic acid:69.4 wt %, a dimer of acrylic acid: 20.9 wt %, maleic anhydride: 6.9 wt%, others: 2.8 wt %) discharged from the bottom of the distillationcolumn in a process for purifying by distillation of acrylic acid,whereby continuous operation for 10 months was accomplished. Whereas,the same operation was carried out by means of the same thin filmevaporator except that no wiper was provided, whereby upon expiration of5 months, the pipeline at the residue discharge port was clogged, andthe operation was obliged to be stopped.

Embodiment e

The present invention provides a method for producing an acrylic ester,wherein acrylic acid is produced by a vapor phase catalytic oxidationreaction of propylene, and the acrylic acid is used as the startingmaterial, wherein as the starting material for producing the acrylicester, acrylic acid is used wherein the concentration of a certainspecific high boiling point impurity is adjusted to be at most aspecific concentration.

Acrylic Ester

The acrylic ester for the present invention may, for example, be methylacrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate,tert-butyl acrylate, 2-ethylhexyl acrylate, isononyl acrylate ormethoxyethyl acrylate.

The process for producing an acrylic ester comprises an esterificationreaction step of reacting acrylic acid with an alcohol such as methylalcohol, ethyl alcohol, n-butyl alcohol, isobutyl alcohol, tertiarybutyl alcohol, 2-ethylhexyl alcohol, isononyl alcohol or methoxyethylalcohol corresponding to each of the above-mentioned acrylic esters, byusing, as a catalyst, an inorganic acid such as sulfuric acid, anorganic acid such as p-toluene sulfonic acid or methane sulfonic acid,or a solid acid such as a cationic ion exchange resin, and apurification step of carrying out washing, liquid-liquid separation,extraction, evaporation, distillation, etc., as unit operations to carryout separation of the catalyst, concentration, purification, etc. of thecrude acrylic ester obtained by the reaction. The starting materialmolar ratio of the acrylic acid to the alcohol in the esterificationreaction, the type and amount of the catalyst to be used, the reactionsystem, the reaction conditions, etc., are optionally set depending uponthe type of the alcohol material. Further, a decomposition step may beprovided wherein β-acryloxypropionic acid and its esters,β-hydroxypropionic acid and its esters, etc., as high boiling pointimpurities produced as by-products of the esterification reaction, arethermally decomposed or catalytically decomposed by means of a catalyst,whereby starting material acrylic acid and alcohol may be recovered.

Acrylic Acid

The acrylic acid to be treated by the present invention is acrylic acidproduced by a vapor phase oxidation reaction of propane or propylene. Inorder to obtain acrylic acid useful as the starting material for anacrylic ester from a gas product of the vapor phase catalytic oxidationreaction of propylene, the acrylic acid-containing gas is contacted withwater to collect acrylic acid in the form of an aqueous acrylic acidsolution. Several methods are available as methods for separating waterfrom such an aqueous acrylic acid solution, but a typical method will beshown as follows. As an example, a method is available wherein acrylicacid is extracted from water by means of an extracting solvent, followedby separating acrylic acid and the extracting solvent by distillation.As another example, a method may be mentioned wherein the aqueousacrylic acid solution is subjected to an azeotropic distillationemploying an azeotropic agent with water, whereby acrylic acid isseparated from water and the azeotropic agent. Further, at that time, byadding an azeotropic agent which is azeotropically distilled with aceticacid as an impurity having a boiling point lower than acrylic acid,water and acetic acid can simultaneously be separated. In a case wherewater is separated and acetic acid is not simultaneously separated, thelow boiling point component may substantially be removed by subjectingthe product to a distillation step to separate low boiling pointimpurities composed mainly of acetic acid. As the starting material fora highly water absorptive resin, high purity acrylic acid is required.Accordingly, a distillation step to remove high boiling point impuritieswill further be required to obtain high purity acrylic acid. On theother hand, as the starting material for an acrylic ester, highly pureacrylic acid is not necessarily required, and it is regarded aseconomically advantageous to use acrylic acid having high boiling pointimpurities not separated, whereby such a distillation step isunnecessary, and the construction costs and the operation costs such asa cost for distillation can be saved.

Details of the Problems to be Solved

If acrylic acid containing high boiling point impurities, is used as astarting material, undesirable polymerization reactions or sidereactions take place, whereby there have been problems, such as cloggingof apparatus such as pipings by a polymer, deterioration of the unitconsumption of main materials such as acrylic acid and an alcohol,deterioration in the quality of the product, etc. A solid polymer formedby an undesirable polymerization reaction tends to accumulate on afilter, a pump strainer, a nozzle, etc., and there has been a problemthat replacement or cleaning is often required, and finally, theoperation is obliged to be stopped due to clogging. Further, a solublepolymer formed by the polymerization reaction tends to bring about atrouble such that an emulsion will be formed in the step of cleaning orseparation of the catalyst, or a problem which leads to a loss of thestarting material acrylic acid.

The undesirable side-reactions include, for example, an acetal-formingreaction, an esterification or ester exchange reaction and an oxidationreaction, and consequently, they mean side-reactions, whereby acrylicacid or an alcohol is consumed uselessly, impurities to contaminate anacrylic ester as the product, are formed, a substance to promote thepolymerization reaction is formed, impurities to create a trouble inoperation will be formed. The present inventors have found that certainspecific impurities are the main factor for the above problems, and onthis basis, have arrived at the present invention.

Specific Impurities

The specific high boiling point impurities which acrylic acid of thepresent invention contains, are four i.e. benzaldehyde, furfural, maleicanhydride and β-acryloxypropionic acid. The concentrations of suchimpurities which are usually contained in acrylic acid prior to removalof the high boiling point impurities, are from 300 to 1000 weight ppm ofbenzaldehyde, from 200 to 500 weight ppm of furfural, from 0.3 to 1.0 wt% of maleic anhydride, and from 1.0 to 3.0 wt % of β-acryloxypropionicacid.

Benzaldehyde and furfural have been found not only to undergo acetalreactions with alcohols in the esterification reaction step thereby toconsume the starting material alcohol uselessly, but also to adverselyaffect the polymerization behavior of the product as the formed acetalsor non-reacted aldehydes are included in the ester product. Further, ithas been found that benzaldehyde and furfural, and their acetals formedby the esterification reaction, are susceptible to oxidation by oxygenadded to prevent polymerization in the purification system, to form aperoxide thereby to accelerate an undesirable polymerization reaction.

It has been found that maleic anhydride not only undergoes anesterification reaction with an alcohol in the esterification reactionstep to form a half ester or a diester thereby to consume the startingmaterial alcohol uselessly, but also be recycled and accumulate in thesystem not only as maleic anhydride, maleic acid and its ester, but alsoas fumaric acid and its ester as isomers thereof, in the processincluding a step of decomposing a high boiling point component. Thisisomerization reaction from maleic acids to fumaric acids, is a reactionwhich takes place since the step of decomposing the high boiling pointcomponent is carried out at a relatively high temperature of from 150 to250° C., and it is a characteristic behavior in a process having a stepof decomposing a high boiling point component. Accumulation of maleicanhydride and fumaric acid, or their esters, has been found to bringabout a decrease in the treating capacity in the step of decomposing thehigh boiling point component, a precipitation trouble of maleic acid orfumaric acid or a problem such that it leads to contamination of theester product.

It has been found that β-acryloxypropionic acid not only undergoes anesterification reaction and an ester exchange reaction with an alcoholin the esterification reaction step thereby to consume the alcoholuselessly, but also β-acryloxypropionic acid itself acceleratespolymerization of acrylic acid.

The upper limit values of the amounts of the high boiling pointimpurities contained in acrylic acid of the present invention are suchthat the total amount of benzaldehyde and furfural is 500 weight ppm,maleic anhydride is 2000 weight ppm, and β-acryloxypropionic acid is1000 weight ppm, preferably 500 weight ppm. Further, the respectivecomponents can be reduced by increasing the precision in distillation(increasing the reflux ratio, or increasing the theoretical platenumber) but practical conditions may be determined taking intoconsideration the balance between the cost required and the effect bysuch reduction.

The lower limit values are considered to be such that the total amountof benzaldehyde and furfural is 50 ppm, maleic anhydride is 50 ppm andβ-acryloxypropionic acid is about 10 ppm.

Method for Producing Specific Acrylic Acid

Several methods may be mentioned as methods for producing acrylic acidof the present invention which contains the specific high boiling pointimpurities at specific concentrations such that the total concentrationof benzaldehyde and furfural is at most 500 weight ppm, theconcentration of maleic anhydride is at most 2000 weight ppm, and theconcentration of β-acryloxypropionic acid is at most 1000 weight ppm,preferably at most 500 weight ppm.

The conventional method for producing acrylic acid without removing thehigh boiling point impurities for the production of an ester, which hasheretofore been commonly employed, is as described above. By using suchacrylic acid as the starting material, the specific acrylic acid of thepresent invention can be produced by employing a unit operation such ascrystallization, distillation or evaporation, or by combining analdehyde-removing reaction employing an amine or hydrazine. However, itis economically preferred to employ a one step flash distillation or avery simple distillation method having a low theoretical plate numberand a low reflux ratio.

Overall Economical Efficiency from Propylene to an Acrylic Ester

When the cost for production of an acrylic ester from propylene as thestarting material, is compared as between a case where the acrylic esteris produced by using acrylic acid as prescribed by the present inventionand a case where the acrylic ester is produced by using acrylic acidcontaining a large amount of high boiling point impurities according tothe conventional technique, an increase in the production cost includingthe installation cost to lower the specific high boiling pointimpurities in acrylic acid to the specific concentration, is notsubstantial, while visible or invisible effects such as improvement inthe unit consumption of the starting materials in the process forproducing the acrylic ester, increase of the production amount of theproduct, improvement of the quality of the product, decrease offrequency of stopping the operation, etc., are much more substantial,whereby the method of the present invention is far advantageous alsofrom the economical efficiency.

Embodiment f

In the present invention, purified (meth)acrylic acid obtained byremoving low boiling point impurities and high boiling point impuritiesfrom a reaction product containing (meth)acrylic acid obtained by vaporphase catalytic oxidation, particularly preferably contains aldehydeshaving boiling points close to (meth)acrylic acid. Further, the bottomfraction from the distillation column is particularly preferably used asa starting material for producing a light (meth)acrylic ester having astandard boiling point lower than (meth)acrylic acid, such as methyl(meth)acrylate or ethyl (meth)acrylate.

Process for Producing Acrylic Acid

The process for producing high purity acrylic acid as an object of thepresent invention, comprises an oxidation step of carrying out a vaporphase catalytic oxidation reaction using propylene and/or propane and/oracrolein as the starting material, a collecting step of contacting anacrylic acid-containing gas from the oxidation step with an absorbingsolvent such as water to collect acrylic acid in the form of an acrylicacid solution, a step of distilling and separating acrylic acid and theabsorbing solvent such as water from this acrylic acid solution, ifnecessary, by means of a suitable azeotropic solvent, a step ofcontinuously distilling and separating acetic acid as a low boilingpoint impurity from acrylic acid, and further a step of distilling andseparating high boiling point impurities, as the basic construction.Here, as the absorbing solvent useful other than water, diphenyl ether,biphenyl or a mixture of diphenyl ether and biphenyl, may be mentionedas a typical example.

Further, also included in the present invention is a method whichcomprises a step of distilling and separating water, acetic acid and thesolvent all at once from the aqueous acrylic acid solution obtained in acase where water is used as the absorbing solvent, or a step ofextracting acrylic acid from the aqueous acrylic acid solution by meansof an extracting solvent such as methyl isobutyl ketone, isopropylacetate, methyl ethyl ketone or toluene and distilling and separatingthe extracting solvent and the remaining water in the extracted acrylicacid. Further, also included in the present invention is a method whichcomprises a step of decomposing a Michael adduct (one having water oracrylic acid added to the double bond of acrylic acid or acrolein)formed as a by-product in the process for production of acrylic acid, astep of further distilling and purifying acetic acid separated bydistillation, or a step of recovering the solvent, etc. by furtherdistilling the aqueous fraction separated by distillation.

Now, as an example of a case where water is used as the absorbingsolvent, an embodiment of the process for purification of an acrylicacid will be described with reference to FIG. 6.

An oxidation reaction gas containing acrylic acid, obtained by vaporphase catalytic oxidation of propylene, propane and/or acrolein by meansof molecular oxygen-containing gas, is introduced into an acrylicacid-collecting column 41 and contacted with water to form a crudeacrylic acid aqueous solution. Here, the oxidation reaction gas containsN₂, CO₂, acetic acid, water, etc., and a part of acetic acid, N₂ and CO₂will be withdrawn as a vent gas from the top of the collecting column41.

The crude acrylic acid aqueous solution from this collecting column 41,is supplied together with an azeotropic agent to a distillation column42 for dehydration, and from the top of the column, an azeotropicmixture comprising water and the azeotropic agent will be distilled, andfrom the bottom of the column, a crude acrylic acid containing aceticacid will be obtained. The azeotropic mixture comprising water and theazeotropic agent distilled from the top of the distillation column 42for dehydration, will be introduced into a decanter 43, wherein it isseparated into an organic phase composed mainly of the azeotropic agentand an aqueous phase composed mainly of water. The organic phasecomposed mainly of the azeotropic agent is, after an addition of apolymerization preventing agent (not shown), returned to thedistillation column 42 for dehydration. On the other hand, the aqueousphase is returned to the acrylic acid-collecting column 41 and used ascollecting water to be contacted with the oxidation reaction gas.Further, a part may be discharged as waste water out of the system, asthe case requires, and water may be supplemented to the water-returningline. There may be a case where in order to recover the azeotropic agentfrom the water in the water-returning line, the water is passed throughan azeotropic agent-recovery column (not shown) and then returned to theacrylic acid-collecting column 41.

The crude acrylic acid containing acetic acid, withdrawn from the bottomof the distillation column 42 for dehydration, is introduced into adistillation column 44 for separating a low boiling fraction in order toremove a low boiling point fraction (low boiling point impurities) suchas remaining acetic acid, and from the top of the column, a low boilingpoint fraction such as acetic acid is separated and removed. The aceticacid from the top of the column contains acrylic acid. Therefore, a partis returned from a reflux tank 44A to the distillation column 44 for alow boiling point fraction, and the rest is returned to the inlet sideof the distillation column 42 for dehydration. Such a low boiling pointfraction containing acetic acid, is separated in the distillation column42 for dehydration and finally discharged as a vent gas out of thesystem via an acrylic acid-collecting column 41.

From the bottom of the distillation column for separating a low boilingpoint fraction 44, acrylic acid containing substantially no acetic acid,will be obtained. Such acrylic acid is introduced into a distillationcolumn for separating a high boiling fraction 45, whereupon heavysubstances (high boiling point impurities) are separated and removed toobtain purified acrylic acid. The bottoms (high boiling pointsubstances) of the distillation column for separating a high boilingpoint fraction 45, are sent to a decomposition reactor (not shown),whereupon acrylic acid, etc. formed by the decomposition reaction willbe recycled for use.

The acrylic acid obtained in the distillation column for separating ahigh boiling fraction 45 is sent to a reflux tank 45A, whereupon a partis returned to the distillation column 45 for separating a high boilingfraction, and the rest is sent to a reactor for converting aldehydes toheavy substances 46, in order to separate aldehydes still contained in avery small amount in this purified acrylic acid by converting them toheavy substances, and an aldehyde-removing agent is added to convert thealdehydes to heavy substances, whereupon such heavy substances will befurther separated and removed in the distillation column forpurification 47. High purity acrylic acid having heavy substances ofaldehydes removed by the distillation column for purification 47, issent to a reflux tank 47A whereupon a part is returned to thedistillation column for purification 47, and the rest is taken out as aproduct.

In the present invention, in the production of such high purity acrylicacid, the bottom fraction (the bottoms) withdrawn from the distillationcolumn for purification 47, i.e. the bottom fraction from thedistillation column for purification 47 to remove aldehydes from thepurified acrylic acid obtained by removing low boiling point impuritiesand high boiling point impurities from the reaction product containingacrylic acid, obtained by vapor phase catalytic oxidation, is sent to aprocess for producing an acrylic ester and is used as the material forproducing an acrylic ester.

Usually, the purified acrylic acid obtained by removing low boilingpoint impurities and high boiling point impurities from a reactionproduct containing acrylic acid, obtained by the vapor phase catalyticoxidation, contains benzaldehyde and furfural having boiling pointsclose to acrylic acid, mainly as an aldehyde component, and theircontents are usually such that each of benzaldehyde and furfural is from20 to 300 weight ppm.

Treating Method for Removal of Aldehydes

The aldehyde-removing agent to remove aldehydes from purified acrylicacid, to be used in the present invention, is not particularly limited,and it may be any conventional removing agent. A typical example of thealdehyde removing agent may, for example, be a hydrazine such asanhydrous hydrazine, hydrated hydrazine or phenyl hydrazine, an aminoacid such as glycine, an amine such as aniline or ethanol amine, ahydrogen sulfite such as sodium hydrogen sulfite, a mercaptan such asoctyl mercaptan, dodecyl mercaptan, hexadecyl mercaptan, octadecylmercaptan or 2-mercaptobenzothiazole, or a combination of a hydrazineand a copper dithiocarbamate.

Such an aldehyde-removing agent will react with aldehydes contained inpurified acrylic acid to form heavy compounds. The reaction conditionsat that time are not particularly limited. After such treatment forheavy compounds, distillation is carried out, whereby high purityacrylic acid which is highly purified and which contains substantiallyno aldehyde component, can be obtained as the top fraction.

In the present invention, the bottom fraction formed by thisdistillation is supplied to a process for producing an acrylic ester. Aswill be mentioned hereinafter, in the process for producing an acrylicester, an acid catalyst is used as a catalyst for the esterificationreaction. Depending upon the type of the above aldehyde-removing agentor the recycling place of the bottom fraction, such an agent or fractionmay sometimes poison or react with the acid catalyst for theesterification reaction. Accordingly, as the aldehyde-removing agent, itmay sometimes be preferred to avoid using a compound which is highlylikely to poison or react with the acid catalyst and which contains anitrogen atom having a basicity (such as an amine or hydrazine) or acompound containing a metal atom (copper or sodium) having a cationexchange ability.

Accordingly, a preferred aldehyde-removing agent for the method of thepresent invention, is one not having a nitrogen atom and a metal atomsimultaneously. Specifically, it may, for example, be an alkylmercaptan,an alkanediol, a mercapto alcohol or a mercapto propionic acid. In acase where such an aldehyde-removing agent is used, the reaction withaldehydes can be effectively accelerated, whereby an acid catalyst maybe employed. As the acid catalyst to be used here, either a solid acidcatalyst such as a strongly acidic ion exchange resin or zeolite, or ahomogeneous acid catalyst such as sulfuric acid or p-toluene sulfonicacid, may be employed.

Acrylic Esters

In the present invention, the acrylic ester for which the bottomfraction from the above-mentioned distillation column for purificationis used as the starting material, may be any acrylic ester obtainable byan esterification reaction of acrylic acid with an alcohol, without anylimitation as to the type of the alcohol. For example, it may be methylacrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate,2-ethylhexyl acrylate, isononyl acrylate or isodecyl acrylate. However,from the after-mentioned reason, an acrylic ester having a standardboiling point lower than acrylic acid, such as methyl acrylate or ethylacrylate, is preferred.

Process for Producing an Acrylic Ester

As the process for producing an acrylic ester to which the presentinvention is applied, one wherein an alcohol is reacted to acrylic acidfor esterification reaction, is common, and it may be either a batchsystem or a continuous system. As the catalyst for the esterificationreaction, it is common to use an acid catalyst. The process forproducing an acrylic ester comprises mainly an esterification reactionstep and a purification step of carrying out washing, extraction,evaporation, distillation or the like as a unit operation to carry outseparation of the catalyst or concentration and purification or the likeof the crude acrylic ester solution obtained by the reaction. Thestarting material molar ratio of acrylic acid to the alcohol in theesterification reaction, the type and amount of the catalyst to be usedfor the reaction, the reaction system, the reaction conditions, etc. aresuitably selected depending upon the type of the alcohol to be used.Further, Michael adducts formed as by-products by a reaction of acrylicacid, an alcohol, water, etc. to acrylic acid or an acrylic ester, willbe concentrated at the bottom of the distillation column for separatinga heavy fraction. Accordingly, such bottoms are subjected to thermaldecomposition or decomposition by means of a Lewis acid or a Lewis basecatalyst, and the obtained useful components are recycled to andrecovered in the reaction step or the purification step. Thedistillation column for separating the heavy fraction may vary dependingupon the type of the acrylic ester to be produced or the processemployed. Usually, it may be one to separate acrylic acid from the heavyfraction, one to separate an acrylic ester from the heavy fraction, orone to separate acrylic acid, an alcohol and an acrylic ester from theheavy fraction. The present invention is applicable to any one of them.

Recycling of the Bottom Fraction from the Distillation Column forPurification to the Process for Producing an Acrylic Ester

In the present invention, the waste liquid obtained by the treatment forremoval of aldehydes from the purified acrylic acid, specifically thebottoms obtained from the distillation column for purification whichseparates by distillation high purity acrylic acid containing noaldehydes and the bottom fraction containing heavy substances convertedfrom aldehydes, after treating purified acrylic acid obtained byremoving low boiling point impurities and high boiling point impuritiesfrom a reaction product containing acrylic acid obtained by vapor phasecatalytic oxidation, with an aldehyde-removing agent to convertaldehydes to heavy compounds, is recycled to the process for producingan acrylic ester. The place to which the bottoms will be recycled, maybe suitably selected depending upon the type of the acrylic ester andits production process. However, since the main component of the bottomsis acrylic acid, it is preferred to recycle the bottoms to theesterification reaction step. However, in a case where a compoundcontaining nitrogen or a metal atom, is used as the aldehyde-removingagent, such a compound adversely affects the catalyst for theesterification reaction, and accordingly, it is preferred to recycle thebottoms to the distillation system or to the step of decomposing theheavy fraction.

Components other than acrylic acid in the bottoms from the distillationcolumn for purification, are, for example, a polymerization inhibitorsuch as hydroquinone or methoquinone (methoxyhydroquinone), a by-productsuch as a dimer (β-acryloxypropionic acid), oligomer or polymer ofacrylic acid and an excessively added remaining aldehyde-removing agent,and a high boiling point compound formed by the reaction of thealdehyde-removing agent with an aldehyde. Among such components in thebottoms, the polymerization inhibitor can effectively be used in theproduction process for an acrylic ester, whereby the amount of thepolymerization inhibitor to be used in the acrylic ester system can bereduced. The dimer, oligomer or polymer of acrylic acid may partially beconverted to the corresponding ester by an alcohol in the reaction step,but the major portion of the heavy fraction is decomposed in thedecomposition step, and can be recovered as acrylic acid or an alcohol.The aldehyde-removing agent and a reaction product of thealdehyde-removing agent with an aldehyde, undergo substantially nochemical reaction in the esterification reaction step, and finallysubjected to the step of decomposing the heavy fraction. In a case wherethe decomposition reaction of this heavy fraction is carried out at avery high temperature or by means of a Lewis acid or a Lewis basecatalyst, such an aldehyde-removing agent or the reaction product of thealdehyde-removing agent with an aldehyde, may cause various reactionssuch as a decomposition reaction, and therefore, a suitablealdehyde-removing agent is selected depending upon the type of the esteror the process.

According to the present invention, it is possible to easily separatesuch side reaction products from the product ester thereby to avoidcontamination of the product ester by such side reaction products.Accordingly, it is preferably applied to a light acrylic ester having astandard boiling point lower than acrylic acid, such as methyl acrylateor ethyl acrylate.

As mentioned above, in a case where the bottom fraction is recycled tothe process for producing acrylic acid, there have been problems such asacceleration of polymerization of acrylic acid by e.g. by-productsformed by the heat decomposition treatment, contamination of the productand coloring. Whereas, according to the present invention, even if thealdehyde-removing agent or the reaction product of the aldehyde-removingagent with an aldehyde, contained in the bottom fraction, may produce asubstance to accelerate polymerization of acrylic acid at the time ofdecomposition reaction of the heavy fraction in the process forproducing an acrylic ester, in the process for producing an acrylicester, the relative concentration of acrylic acid in the system is low,whereby problems such as acceleration of polymerization of acrylic acid,contamination of the product and coloration, by by-products, etc., canbe substantially reduced.

In the present invention, it is possible to send the waste liquidobtained in the distillation column for purification of high purityacrylic acid, as it is, to the process for producing an acrylic ester,without treatment thereof, thereby to effectively use acrylic acidcontained therein, as a material for production of an acrylic ester,whereby it is not necessary to set the conditions to bring the acrylicacid concentration in the bottoms to be sufficiently low, as thedistillation condition for the distillation column for purification forseparation by distillation of high purity acrylic acid, and distillationcan be carried out even under such a condition that the acrylic acidconcentration in the bottoms is at least 70 wt %, such as from 70 to 95wt %. And, in such an acrylic acid concentration, there will be noclogging of the piping system of the distillation column forpurification, and continuous operation can be continued for a longperiod of time.

In the foregoing, the production of high purity acrylic acid has beendescribed, but the present invention can be applied in the same manneralso in the process for producing high purity methacrylic acid by avapor phase catalytic oxidation reaction of isobutylene and/or t-butylalcohol.

Examples

Now, the present invention will be described in further detail withreference to Examples, but it should be understood that the presentinvention is by no means restricted to the following Examples. Further,in the following Examples, analyses of the respective compositions werecarried out by gas chromatography (GC14A, manufactured by ShimadzuCorporation). However, maleic acid is converted to maleic anhydride inthe step of gas chromatography analysis, and the contents of the twocannot be specified. Accordingly, in the following, the total content ofmaleic acid and maleic anhydride is represented by the content of maleicacids.

Example a1

In accordance with the flow sheet shown in FIG. 1, acrylic acid of thepurity for an ester, and acrylic acid of the purity for a highly waterabsorptive resin are produced. To the first distillation column, crudeacrylic acid (purity: 93.8 wt %) is supplied at a rate of 11053 kg/hr,and the top fraction from the third distillation column is supplied at arate of 2390 kg/hr. As the first distillation column, a distillationcolumn equipped with dual flow trays having a theoretical plate numberof 7 plates is used, and it is operated at a reflux ratio of 0.7 at abottom temperature of 80° C. under a top pressure of 20 Torr. From thetop of the first distillation column, 10460 kg/hr (purity: 99.8 wt %) ofthe top fraction is obtained, and 6160 kg/hr is used for an ester and4300 kg/hr of the rest is, after mixed with 10 kg/hr ofn-dodecylmercaptan as an aldehyde-removing agent, passed through apacked column of a sulfonic acid type cation exchange resin (DIAIONPK-216H, DIAION is a registered trademark of Mitsubishi ChemicalCorporation) and then supplied to the second distillation column. As thesecond distillation column, a packed column having a theoretical platenumber of 9 plates is used, and it is operated at a reflux ratio of 1 ata bottom temperature of 70° C. under a top pressure of 16 Torr, toobtain 3897 kg/hr of acrylic acid having a purity of 99.94 wt % from thetop. This acrylic acid adequately satisfies the quality required foracrylic acid for a highly water absorptive resin.

2983 kg/hr of the bottoms from the first distillation column and 413kg/hr of the bottoms from the second distillation column are puttogether and supplied to the third distillation column. As the thirddistillation column, a vertical thin film evaporator is used, and it isoperated under a pressure of 70 Torr at a flow out gas temperature of110° C. From the top, 2390 kg/hr of acrylic acid having a purity of 89.0wt % is recovered and as mentioned above, supplied to the firstdistillation column. 1006 kg/hr of the bottoms from the thirddistillation column is supplied to a thermal decomposition column andthermally decomposed at a bottom temperature of 180° C. under a toppressure of 500 Torr for a retention time of 3 hours, whereby from thetop, 664 kg/hr of a distillate having an acrylic acid purity of 91.1 wt% is taken out and returned to a low boiling point component-removingstage in the preliminary purification step. 342 kg/hr of the bottomsfrom the thermal decomposition column is supplied to an incinerationapparatus. In this manner, purification of acrylic acid can be carriedout constantly over a long period of time.

Example b1

To a crude acrylic acid obtained by vapor phase catalytic oxidation andcontaining as impurities 239 pm (weight) of furfural, 238 ppm (weight)of benzaldehyde and 3300 ppm (weight) of maleic anhydride, hydrazinehydrate was added in an amount equal by mol to the total molar amount ofaldehydes and maleic acids, and the mixture was passed through a tubularreactor at a flow rate of 5000 kg/hr as the total liquid amount at areaction temperature of 20° C. for a retention time of 2 hours. Afterthe treatment for removal of aldehydes and maleic acids, the reactionsolution was taken out from the piping and found to be in a slightlyyellow slurry state. This slurry was heated by means of a heat exchangerso that the internal temperature became 65° C. The reaction solutionprior to the supply to the distillation apparatus was a yellowish browntransparent liquid, and no precipitation of solid was observed. Thisyellowish brown transparent liquid was sent, as it was, to the packedcolumn distillation apparatus and subjected to continuous distillation.Here, the heating time in the heat exchanger was about 1 minute whichcorresponds to the flowing time of the reaction solution.

The continuous distillation was carried out at a bottom temperature of74° C., whereby 99 wt % of the supplied liquid was continuouslydistilled, and part of the distillate was, as a reflux liquid,introduced into the column from the top at a reflux ratio of 1.0.Further, at the time of the continuous distillation, as a polymerizationinhibitor, methoquinone (methoxy hydroquinone) corresponding to 10weight ppm to the liquid amount introduced into the distillation column,was introduced into the column, as dissolved in the reflux liquid.

The concentrations of maleic acids and aldehydes such as furfural,benzaldehyde, etc. in the purified acrylic acid obtained as a distillatefrom the top of this distillation column, were not more than 1 ppm,respectively, and under this condition, it was possible to carry outcontinuous distillation constantly for 5 months.

Comparative Example b1

Distillation was carried out under the same conditions as in Example b1except that the feeding temperature of the reaction solution to thedistillation column was changed to 50° C. A part of the reactionsolution prior to feeding into the distillation column was withdrawn andfound to be still a slightly yellow slurry, and it was sent, as it was,to the continuous distillation apparatus, whereby upon expiration of 3months, it became impossible to continue the distillation due to anincrease in the pressure difference in the column.

Comparative Example b2

Distillation was carried out under the same conditions as in Example b1except that the feeding temperature of the reaction solution to thedistillation column was changed to 85° C. As a result, in the purifiedacrylic acid obtained as a distillate from the top of the distillationcolumn, the concentration of furfural was 5 ppm, the concentration ofbenzaldehyde was 10 ppm and the concentration of maleic acids was 160ppm, and it was impossible to use it as high purity acrylic acid.

Examples c1 to c4

A distillation column made of glass was used wherein in the interior ofthe column having an inner diameter of 50 mm and a length of 650 mm, acoil pack made of SUS316 and having a diameter of 3 mm, was packed at aheight of 300 mm at the enriching section and to a height of 300 mm atthe stripping section, and a three necked flask of 1000 cc was providedat the bottom of the column. The main body of the column was covered byan electric heater to avoid condensation on the wall surface of thecolumn, and also at a lower portion of the three necked flask, anelectric heater was provided for heating, whereby distillation of crudeacrylic acid was carried out. As the crude acrylic acid monomer, amixture was used which contained 98.5 wt % of acrylic acid, 0.3 wt % ofmaleic acid, 0.276 wt % of a dimer of acrylic acid, 0.02 wt % offurfural and 0.004 wt % of benzaldehyde.

Prior to feeding the crude acrylic acid monomer into the distillationcolumn, hydrazine hydrate, copper dibutyldithiocarbamate, copperacrylate, etc. were mixed in the ratio as identified in Table 1. Here,the copper acrylate was one prepared by dissolving cupric carbonate inacrylic acid, and the mixing was carried out at 20° C. for 30 minutes.

Prior to initiation of the operation, 800 g of acrylic acid having apurity of 99.8 wt % containing 200 weight ppm of methoquinone wassupplied to the distillation column to wet the surface of the packingmaterial in the column. The distillation feed liquid (the crude acrylicacid) containing the hydrazine compound and the copper compounds in theabove identified ratio, was supplied from the center portion of thecolumn, while methoquinone was supplied to the reflux tank at the top sothat its concentration in the highly concentrated acrylic acid distilledfrom the top would be 200 weight ppm. The liquid flowed down to thebottom, was withdrawn out of the system. After supplying thedistillation feed liquid to the distillation column, when a liquidsurface was confirmed at the bottom, heating from the bottom of thecolumn was initiated.

Continuous operation was carried out under the respective conditions asidentified in Table 1. From the top of the column, high purity acrylicacid having an acrylic acid purity of at least 99.5 wt % and furfuraland benzaldehyde being not more than 1 weight ppm, respectively, wasobtained. The feeding rate of the distillation feed liquid during thesteady operation was 265 g/hr, the withdrawn amount of the high purityacrylic acid was 95 wt % of the feeding amount of the distillation feedliquid, and further, from the bottom, withdrawal of the bottoms wascontinuously carried out so that the amount of the liquid in the threenecked flask would be constant.

Upon expiration of 48 hours, the operation was terminated, and theinterior of the distillation column was inspected, whereby no formationof a polymer was observed at any place.

Examples c5 to c9

In Examples c1 to c4, without mixing copper compounds to the crudeacrylic acid monomer, only hydrazine hydrate was mixed in the sameamount to prepare a distillation feed liquid. Instead, copperdibutyldithiocarbamate and copper acrylate were mixed to the top liquidand supplied via a reflux line into the distillation column. Then,distillation was carried out in the same manner as Examples c1 to c4.Upon expiration of 48 hours, the operation was terminated, and theinterior of the distillation column was inspected, whereby no formationof a polymer was observed at any place. The results are shown in Table1.

Comparative Examples c1 to c7

In Examples c1 to c4, without adding both copper dibutyldithiocarbamateand copper acrylate to the crude acrylic acid monomer, only either onewas mixed to prepare a distillation feed liquid. Further, in oneexample, no hydrazine compound was added. Then, distillation was carriedout in the same manner as in Examples c1 to c4, but turbidity formed inthe bottoms (Comparative Examples c1, c5, c6 and c7), precipitation of apolymer was observed in the three necked flask at the bottom(Comparative Examples c1, c2, c4, c5, c6 and c7), and precipitation ofpolymer was observed also at the stripping section of the distillationcolumn (Comparative Examples c2, c3, c4 and c7). The results are shownin Table 2.

TABLE 1 Example Nos. 1 2 3 4 5 6 7 8 9 Hydrazine Concentration 1650 16501650 1650 1650 1650 1650 1650 1650 hydrate in distillation feed liquid:weight ppm Copper Same as above 40 20 40 40 20 60 40 40 40dibutyldithio- carbamate Copper Same as above 40 20 40 40 20 60 40 40 40acrylate Hydroquinone Same as above — — 300 — — — 300 300 —Phenothiazine Same as above — — — 150 — — 150 150 Top pressure kPa 8.12.8 10.1 10.1 2.8 11.3 12.7 10.1 10.1 Bottom ° C. 90 80 95 95 80 100 10595 95 temperature Bottom kPa 10.1 6.5 12.1 12.1 6.5 13.3 14.4 12.1 12.1pressure Furfural in Weight ppm 1> 1> 1> 1> 1> 1> 1> 1> 1> top liquidBenzaldehyde Weight ppm 1> 1> 1> 1> 1> 1> 1> 1> 1> in top liquid Resultsof — No No No No No No No No No inspection of Polymer Polymer PolymerPolymer Polymer Polymer Polymer Polymer Polymer distillation columnafter continuous operation for 48 hours

TABLE 2 Comparative Example Nos. 1 2 3 4 Hydrazine Concentration 16501650 1650 1650 hydrate in distillation feed liquid: weight ppm CopperSame as above — 40 40 40 dibutyl- dithio- carbamate Copper Same as above40 — — — acrylate Hydroquinone Same as above — — 300 — PhenothiazineSame as above — — — 150 Top pressure kPa 8.1 8.1 10.1 10.1 Bottom ° C.90 90 95 95 temperature Bottom kPa 10.1 10.1 12.1 12.1 pressure Furfuralin Weight ppm 1> 1> 1> 1> top liquid Benzaldehyde Weight ppm 1> 1> 1> 1>in top liquid Results of Operation 48 Terminated Terminated Terminatedinspection of time hours after after after distillation 5 hrs 9 hrs 10hrs column Main body No Polymer Polymer Polymer of distillation Polymerobserved observed observed column at at at stripping stripping strippingsection section section Bottom of Polymer Polymer No Polymer columnobserved observed Polymer observed (three-necked flask) BottomsTurbidity No No No observed Polymer Polymer Polymer Comparative ExampleNos. 5 6 7 Hydrazine Concentration 1650 1650 0 hydrate in distillationfeed liquid: weight ppm Copper Same as above — — 40 dibutyl- dithio-carbamate Copper Same as above 40 40 — acrylate Hydroquinone Same asabove 300 — 300 Phenothiazine Same as above — 150 — Top pressure kPa10.1 10.1 10.1 Bottom ° C. 95 95 95 temperature Bottom kPa 12.1 12.112.1 pressure Furfural in Weight ppm 1> 1> 100 top liquid BenzaldehydeWeight ppm 1> 1> 4 in top liquid Results of Operation 48 hours 48 hours48 hours inspection of time distillation Main body No No Polymer columnof distillation polymer polymer observed column at Stripping sectionBottom of Polymer Polymer Polymer column observed observed observed(three-necked flask) Bottoms Turbidity Turbidity Turbidity observedobserved observed

Example c10

In Example c8, instead of the distillation column made of glass andequipped with a three necked flask, a distillation column made ofstainless steel (SUS316) and having an irregular packing material (IMTP)manufactured by Norton Company packed in 8 m in the interior having aninner diameter of 1100 mm and a length of 20000 mm and having 9perforated plates beneath the packing material, was used, and thedistillation feed liquid was supplied at a rate of 1300 kg/hr. Then, theoperation was initiated, and the continuous operation was carried out,in the same manner as in Example 8.

Constant operation was carried out without any change in the pressuredifference in the distillation column, whereby from the top of thecolumn, 95% of the supplied amount of the distillation feed liquid, waswithdrawn to obtain high purity acrylic acid having an acrylic acidpurity of at least 99.5 wt % and furfural and benzaldehyde being notmore than 1 weight ppm, respectively. Upon expiration of 3 months, theoperation was terminated, and the interior of the distillation columnwas inspected, whereby no formation of a polymer was observed at anyplace.

Example e1 Preparation of Acrylic Acid

Using as a feed material acrylic acid from the bottom of an azeotropicdistillation column having an azeotropic agent, water and acetic acidremoved in a plant for producing acrylic acid, the first stagecontinuous flash distillation was carried out to obtain acrylic acid ofthe present invention as a distillate. The compositions of the feedmaterial and the distillate were analyzed by gas chromatography, and theresults are shown in Table 4. The flash distillation column was operatedunder a pressure of 10 kPa at a temperature of 80° C., and the operationwas carried out so that the flash ratio to the feed liquid became 40%.

TABLE 4 Compositions of the feed material for production of acrylic acidand the product acrylic acid of the present invention Feed materialDistillate Purity of acrylic acid 96.00 wt % 99.70 wt % High boilingpoint impurities Benzaldehyde 421 weight ppm 279 weight ppm Furfural 242weight ppm 203 weight ppm Maleic anhydride 0.69 wt % 0.157 wt %β-acryloxypropionic 2.26 wt % 290 weight ppm acid Total of other 0.96 wt% 760 weight ppm impurities

Using the distillate in the above Table as a starting material (materialA) of the present invention and the feed material in the above Table asa starting material (material B) of Comparative Examples, production ofthe following esters was carried out.

Example e2 Production of Methyl Acrylate

Acrylic acid recovered via an acid separation column and a heavyfraction separation column using acrylic acid of material A as thestarting material, and fresh methyl alcohol and methyl alcohol recoveredfrom an alcohol recovery column were put together and continuously fedat a rate of 1300 kg/hr to and reacted in an esterification reactorpacked with 1400 l of H type strongly acidic ion exchange resinDIA-ionPK-208 (manufactured by Mitsubishi Chemical Corporation). Thereaction conditions were such that methyl alcohol:acrylic acid=1.0:1.0(molar ratio), and the temperature was 80° C. The reaction product wascontinuously supplied to an acid separation column operated at the toppressure of 27 kPa at a bottom temperature of 93° C. at a toptemperature of 41° C. in a reflux ratio (R/D) of 1.0. The crude methylacrylate obtained from the top of the column was sent to a productcolumn via an alcohol extraction column and a low boiling componentseparation column, to obtain 860 kg/hr of methyl acrylate. The fractionrich in acrylic acid at the bottom was subjected to a heavy fractionseparation column, whereby acrylic acid was recovered from the top, anda heavy fraction at the bottom was subjected to a thermal decompositionreaction at a temperature of 200° C. in a high boiling point componentdecomposition reactor, to recover useful components.

As a result of continuous operation for 3 months, the productivity ofthe product was constantly maintained at 20.6 ton/day. Further, it wasnot necessary to clean or replace the strainer of the withdrawal pump atthe bottom of the acid separation column. Further, during this period,the conversion in the heavy substance decomposition step (the conversionof methyl β-acryloxypropionate) was maintained to be 62%.

Comparative Example e1

Using acrylic acid of material B as the starting material, operation wascontinued for 3 months under the same conditions as in Example e2. Theproductivity of the product at the initial stage of the operation was20.4 ton/day, but 3 months later, it decreased to 19.9 ton/day. Further,it was necessary to switch, disassemble and clean the strainer of thepump at the bottom of the acid separation column twice because of anincrease in the pressure difference due to clogging by a solidsubstance. Further, the conversion in the heavy substance decompositionstep decreased from 60% at the initial stage to 53% upon expiration of 3months.

Comparative Example e2 PRODUCTION of 2-ETHYLHEXYL ACRYLATE

Using material B, the product 2-ethylhexyl acrylate was produced byesterification with 2-ethylhexyl alcohol using p-toluene sulfonic acidas a catalyst. A polymerizability test (*) of the product was carriedout, whereby the introduction period for polymerization was 18 minuteson average. (*) Tests for judging the polymerizability

Example e3

Using material A, 2-ethylhexyl acrylate was produced in the same manner.A polymerizability test of the product was carried out, whereby theintroduction period for polymerization was 15 minutes on average.

10 ml of the product 2-ethylhexyl acrylate (containing 15 ppm ofp-methoxyphenol as the polymerization inhibitor) was put into acontainer having an internal capacity of 30 ml and equipped with athermocouple and a gas-supply tube and immersed in an oil bath whileblowing air into the container at a rate of 30 ml/min and the internaltemperature of the container was raised to 150° C. The internaltemperature of the container was once reached equilibrium at 150° C.,then, when polymerization started, the temperature started to rise bythe polymerization heat, whereby the time from the start of rising ofthe internal temperature to a point when it reached 155° C., was takenas the introduction period.

Example f1 A Case Wherein the Bottoms from the Distillation Column forPurification were Recycled to the Process for Producing Methyl Acrylate

2.1 kg/hr of 1,2-ethanedithiol as an aldehyde-removing agent, was mixedto 1200 kg/hr of acrylic acid (purity: 99.8 wt %) containing 210 weightppm of furfural and 100 weight ppm of benzaldehyde, and the mixture waspassed through a reaction column packed with 600 l of H type stronglyacidic ion exchange resin (SK-104, manufactured by Mitsubishi ChemicalCorporation) at a temperature of 90° C. This aldehyde removal treatedliquid was distilled by a distillation column for purification operatedat a plate number of 10 plates at a reflux ratio of 1 under a toppressure of 27 kPa. At that time, to the feed liquid to the distillationcolumn for purification, an acrylic acid solution containinghydroquinone at a concentration of 10 wt % as a polymerizationinhibitor, was added at a rate of 12 kg/hr, and to the top of thedistillation column, hydroquinone monomethyl ether (methoquinone) as apolymerization inhibitor was injected so that the concentration in thedistilled high purity acrylic acid would be 200 weight ppm.

As a result, from the bottom of the distillation column forpurification, the bottoms having a composition comprising 78 wt % ofacrylic acid, 12 wt % of β-acryloxypropionic acid and 10 wt % of otherheavy substances, was obtained at a rate of 25 kg/hr. Such bottoms weresupplied in the entire amount to an esterification reactor in theprocess for producing methyl acrylate.

From the top of the distillation column for purification, high purityacrylic acid having a purity of 99.92 wt % was obtained at a rate of1190 kg/hr. The concentrations of furfural and benzaldehyde in this highpurity acrylic acid were measured by gas chromatography, whereby eachwas not more than the detectable limit (1 weight ppm).

The operation of supplying the entire amount (25 kg/hr on average) ofthe bottoms obtained to the esterification reactor in the process forproducing methyl acrylate, while maintaining the above conditions, wascontinued for 2 months, whereby there was no trouble of clogging in thewithdrawal pipe of the bottoms or in the feeding pipe to the reactor formethyl acrylate.

Further, also in the process for producing methyl acrylate, noabnormality such as polymerization trouble or clogging trouble wasobserved, and no change in the quality of the product of methyl acrylatewas observed, by the operation by supplying the bottoms, as comparedwith the case of the operation carried out without supplying thebottoms.

Further, unit consumption of the starting materials in the process forproducing methyl methacrylate for this period of 2 months is comparedwith the unit consumption at the time when no bottoms were supplied likein the conventional process, whereby it was calculated that the decreaseof acrylic acid was about 19 kg/hr. Here, during this period, it waspossible to carry out the operation by reducing the feeding amount ofhydroquinone in the purification process for methyl acrylate by 1 kg/hras compared with a case where no bottoms were supplied.

Comparative Example f1 A Case Wherein the Bottoms from the DistillationColumn for Purification were Disposed

The aldehyde treatment and distillation of high purity acrylic acid werecarried out in the same manner as in Example f1 except that operationwas carried out by reducing the withdrawal amount of the bottoms fromthe distillation column for purification. Namely, in this ComparativeExample, as the bottoms from the distillation column for purificationwere disposed, the operation was carried out so that acrylic acid wasrecovered as far as possible, and distillation was carried out so thatthe withdrawal amount of the bottoms would be 15 kg/hr, but cloggingoccurred in the withdrawal pipe of the bottoms, and it was impossible tocontinue the operation. The composition of the bottoms at that time was51 wt % of acrylic acid, 20 wt % of β-acryloxypropionic acid and 29 wt %of other heavy substances, etc.

Comparative Example f2 A Case Wherein the Bottoms from the DistillationColumn for Purification was Recycled to the Process for ProducingAcrylic Acid

The aldehyde treatment and distillation of the high purity acrylic acidwere carried out in the same manner as in Example f1. However, 25 kg/hrof the bottoms from the distillation column for purification weresupplied to the decomposition reactor for the heavy fraction in theprocess for producing acrylic acid, and continuous operation for 1 monthwas carried out. The decomposition reaction in the decomposition reactorfor a heavy fraction was carried out in the absence of a catalyst andconducted in a reactive distillation system by connecting a distillationcolumn to the upper part of the decomposition reactor. The conditionsfor the decomposition reaction were such that the retention time basedon the withdrawal liquid was 1 hour, the temperature was 190° C., andthe pressure was 100 kPa. The distillate obtained from the decompositionreaction distillation was supplied to a dehydration column, but in thisdehydration column, the pressure difference between the top and thebottom of the column increased by 0.5 kPa upon expiration of 1 month.

Further, in a case where the operation was carried out in the samemanner as in Example f1, no increase in the pressure difference in thedehydration column was observed after one month of operation, and inthis Comparative Example, it is considered that polymerization ofacrylic acid was accelerated due to decomposition by-products of thebottoms, whereby an increase in the pressure difference in thedehydration column was brought about.

INDUSTRIAL APPLICABILITY

a. According to the present invention, from crude (meth)acrylic acid,both (meth)acrylic acid of the purity for an ester and (meth)acrylicacid of the purity for a highly water absorptive resin can efficientlybe produced. In the present invention, only (meth)acrylic acid directedto the purity for a highly water absorptive resin, is treated by analdehyde-removing agent, whereby the removing agent can be saved, and asthe amount of the liquid to be treated is small, the removal operationis easy. Further, also in a case where only (meth)acrylic acid for ahighly water absorptive resin is to be produced, the amount of theliquid to be treated in the second distillation column is small ascompared with in the first distillation column, whereby the merit of thepresent invention to supply an aldehyde-removing agent to the seconddistillation column will continuously be maintained. Further, in thepresent invention, the bottoms from the first distillation column andthe second distillation column are not discharged out of the system asthey are, but they are distilled in the third distillation column, sothat acrylic acid in these bottoms is recovered as much as possible andrecycled to the first distillation column, whereby the obtainable ratioof the purified acrylic acid from the supplied crude acrylic acid can bemaintained at a high level.

b. According to the method for producing (meth)acrylic acid according tothe present invention, even if impurities such as maleic acid and/orcitraconic acid are contained in a relatively large amount in the crude(meth)acrylic acid obtained by vapor phase catalytic oxidation, itbecomes possible to produce high purity (meth)acrylic acid having anextremely small content of impurities constantly for a long period oftime by suppressing formation of sludge during the purification bycontinuous distillation, and thus, its industrial value is significant.

c. Impurities contained in the crude (meth)acrylic acid obtained byvapor phase catalytic oxidation can easily be removed by thedistillation method. By suppressing polymerization of a (meth)acrylicacid monomer during the distillation, constant operation for a longperiod of time can be carried out, and thus, its industrial value issignificant.

d. According to a thin film evaporator according to the presentinvention, it is possible to suppress formation of a polymer in theinterior of the evaporator, and even if a polymer or precipitate isformed, it is possible to prevent deposition thereof, whereby constantoperation for a long period of time of the thin film evaporator ispossible. It is thereby possible to stabilize the production. Thus, thethin film evaporator according to the present invention can be said tobe an instrument very useful for industrial purpose.

e. By using, as the starting material, acrylic acid having the specificimpurities controlled to the specific concentration according to thepresent invention, it is possible to avoid conventional problems such asclogging of apparatus such as pipings by a polymer, deterioration ofunit consumption of the starting materials, deterioration of the qualityof the product, etc., and at the same time, it is possible to provide anindustrially advantageous method for producing an acrylic ester, whichis excellent also in the economical efficiency.

f. According to the present invention, it is possible to produce highpurity (meth)acrylic acid purified to a high level by efficiently andsimply removing aldehydes contained in (meth)acrylic acid, and at thesame time, it is possible to recycle the waste liquid other than thehigh purity (meth)acrylic acid fraction, which forms by such treatmentof aldehydes without any special treatment, to a process for producing a(meth)acrylic ester, in the entire amount as it is, and to effectivelyuse it, whereby the following excellent effects can be obtained.

{circle around (1)} It is possible to avoid conventional problems suchas a polymerization trouble in the purification step for (meth)acrylicacid caused by an aldehyde-removing agent, or contamination orcoloration of the product, etc. which used to be caused by recycling ofthe waste liquid to the process for producing (meth)acrylic acid.

{circle around (2)} Treatment of the waste liquid which used to berequired, will be unnecessary.

{circle around (3)} Useful components such as (meth)acrylic acid and adimer of (meth)acrylic acid which used to be disposed, can be recoveredand reused, whereby unit consumption of the starting material will beimproved.

{circle around (4)} (Meth)acrylic acid in the waste liquid can berecovered without being lost, whereby it is possible to increase theconcentration of (meth)acrylic acid in the bottoms in the distillationcolumn for purification to separate high purity (meth)acrylic acid andto avoid a trouble such as clogging in the distillation system.

{circle around (5)} It is possible to effectively use a polymerizationinhibitor in the waste liquid, which used to be disposed, whereby theamount of the polymerization inhibitor to be used in the process forproducing a (meth)acrylic ester, can be reduced.

The entire disclosures of Japanese Patent Application No. 2001-332008filed on Oct. 30, 2001, Japanese Patent Application No. 2001-360437filed on Nov. 27, 2001, Japanese Patent Application No. 2001-368858filed on Dec. 3, 2001, Japanese Patent Application No. 2001-373671 filedon Dec. 7, 2001, Japanese Patent Application No. 2002-003590 filed onJan. 10, 2002 and Japanese Patent Application No. 2002-131675 filed onMay 7, 2002 including specifications, claims, drawings and summaries areincorporated herein by reference in their entireties.

1. A method for producing (meth)acrylic acid, comprising: feeding acrude (meth)acrylic acid obtained by vapor phase catalytic oxidation toa distillation column to continuously distil and purify it in thepresence of a hydrazine, wherein the hydrazine is added to the crude(meth)acrylic acid prior to feeding to the distillation column, and thecrude (meth)acrylic acid having the hydrazine added thereto is heated toa feeding temperature of from 60° C. to 80° C. and then fed to thedistillation column at the feeding temperature.
 2. The method accordingto claim 1, wherein the feeding temperature is from 60° C. to 75° C. 3.The method according to claim 1, wherein the feeding temperature is from62° C. to 80° C.
 4. The method according to claim 1, wherein the feedingtemperature is from 62° C. to 75° C.